Aeolus

“ As there, the primitive involution is merely represented by a split separating the yolk and the germinal disc, and on the dorsal side alone is there a true cellular wall for this split, and at the dorsal mouth of the split the alimentary epithelium becomes continuous with the epiblast.
The segmentation cavity has become diminished, and round the yolk the epiblast, accompanied by a layer of mesoblast, is commencing to grow. In this growth all parts of the blastoderm take a share except that part where the epiblast and hypoblast are continuous. ”

“ If we open up one of these little warts, very carefully with our mounted needles, we shall find that each little pore opens into a cavity, within which we can find two kinds of structures, hidden amongst a number of hair like growths. We shall see a number of dark, oval bodies, at the base of the hairs, these are the egg cells; more careful search will show us a number of much branched structures also partly concealed by the hairs, these are the male organs of the plants. ”

“ All viviparous quadrupeds, then, are furnished with an oesophagus and a windpipe, situated as in man; the same statement is applicable to oviparous quadrupeds and to birds, only that the latter present diversities in the shapes of these organs. As a general rule, all animals that take up air and breathe it in and out are furnished with a lung, a windpipe, and an oesophagus, with the windpipe and oesophagus not admitting of diversity in situation but admitting of diversity in properties, and with the lung admitting of diversity in both these respects. ”

“ But in those animals that have but little superfluous fluid to excrete, such as birds and fishes, the spleen is never large, and in some exists no more than by way of token. So also in the oviparous quadrupeds it is small, compact, and like a kidney. For their lung is spongy, and they drink but little, and such superfluous fluid as they have is applied to the growth of the body and the formation of scaly plates, just as in birds it is applied to the formation of feathers. ”

“ Embryology.—The first appearance of a vascular system is outside the body of the embryo in the wall of the yolk sac, that is to say, in the mesoderm or *the middle one of the three embryonic layers. The process is a very early one and in the chick is seen to begin at the end of the first day of incubation. ”

“ As only one areola belongs to each cell, and as in many cases where it exists in the common cells of the epidermis, it is also visible in the cutaneous glands or stomata, and in these is always double—one being on each side of the limb—it is highly probable that the cutaneous gland is in all cases composed of two cells of peculiar form, the line of union being the longitudinal axis of the disk or pore. ”

“ Histologically the ovum, or egg-cell, is nearly identical in all animals, whether vertebrate or invertebrate. Considered as a cell it is of large size, but actually it is not more than 1/100, and may be less than 1/200 of an inch in diameter. In man, as in most mammals, it is about 1/120. It is a more or less spherical body, presenting a thin transparent envelope, called [121] the zona pellucida, which contains—first, the protoplasmic cell-substance or “yolk,” within which lies, second, the nucleus or germinal vesicle, within which again lies, third, the nucleolus or germinal spot. ”

“ The more important points of distinction consist in the conformation of the opening of the cell, and in the position of the avicularium when the latter organ is present. The lower half of what would otherwise be the oral opening of the cell is filled up by a thin plate of calcareous matter, granulated on the surface, and by which the actual opening is rendered more or less subtriangular, the mouth being placed just below the apex of the triangle. The margin of the opening is considerably raised, especially at the oval end, so that the opening appears to be situated in a deep depression. ”

“ In the development the segmentation is complete, a gastrula is formed by invagination, the nervous system is formed from the dorsal epiblast, the notochord from the hypoblast, and the mesoderm arises from metameric coelomic pouches. The body-cavity is an enterocoele. The gill-slits are at first perforations of the body-wall opening from the pharynx to the exterior, which later become enclosed by the development of the atrium. ”

“ There are three pairs of spermathecae situated in segments III-V, a testis in V and an ovary in VI. There are a clitellum and sperm ducts which though like nephridia have a larger funnel and a less complexly wound duct. This family consists of only one well-known genus, Aeolosoma, which contains several species. They are minute worms with coloured oil drops (green, olive green or orange) contained in the epidermis. The nervous system is embedded in the epidermis, and the pairs of ganglia are separated as in Serpula, &c.; each pair has a longish commissure between its two ganglia. ”

“ Within the yolk the nuclei of some “yolk cells” can be distinguished.
Germinal Layers and Food-Canal.—The embryo begins to develop as an elongate, thickened, ventral region of the blastoderm which is known as the ventral plate or germ band. Along this band a median furrow appears, and a mass of cells sinks within, the one-layered germ band thus becoming transformed into a band of two cell-layers (fig. 15) . In some cases the inner layer is formed not by invagination but by proliferation or by delamination. ”

“ If a male, there will be two little oval-shaped bodies of a dull white or light yellow colour, one on each side of the backbone and at the upper end of the kidneys. If a female, these two little organs will not be there, but a little lower down there will be a string or bundle of eggs, very, very small in some seasons, but well developed in the breeding seasons. ”

“ It is closed in above and at the sides by the hypoblast, and below by the yolk. In its anterior part a floor is commencing to be formed by a growth of cells from the walls of the two sides. The cells for this growth are formed around the nuclei of the yolk; a feature which recalls the fact that in Amphibians the ventral wall of the alimentary cavity is similarly formed in part from the so-called yolk cells. ”

“ Upon removal of the prosomatic carapace of Limulus, a large brownish glandular-looking mass is seen, in which, if it happens to be a female, masses of ova are very conspicuous. This mass is composed of two separate glands, the generative glands and the hepatico-pancreatic glands—the so-called liver—and surrounds closely the central nervous system and the alimentary canal. ”

“ The purpose of the hairs, by the way, is to keep the little chamber moist, when the plant is left high and dry. If we watch the pores of the other warts carefully we may be fortunate enough to see the process of increase taking place, for it occurs outside the plant and not within the chamber. The egg cell divides into two and its contents pass out of the chamber by way of the pore; each cell of the male organs gives rise to sixty-four oval little structures, each provided with a pair of minute whip-like threads by means of which it swims from the chamber and goes in search of the egg cells. ”

“ The median furrow expands upon it. The cephalic borders are next united by a thin layer of epithelial cells above the furrow, converting it into a cavity, or ventricle. The myelonal furrow is similarly covered by a layer, uniting the lateral columns. The embryonal trace becomes longer, narrower, and bends round the vitellus. A layer of epithelial cells forms a network over the whole dorsal surface of the embryo. ”

“ In the lower vertebrates, the only glandular masses apart from the cell-lining of the body-cavity itself, which give rise to leucocyte-forming tissue, are these segmental branchial glands, or possibly also the modified post-branchial segmental glands, known as the head-kidney in Teleostea, etc. ”

“ The white-yolk cells near the line of junction can frequently be seen to possess nuclei. At first the hypoblast appears to end abruptly against the white yolk; this state of things, however, soon ends, and there supervenes a complete and unbroken continuity between the hypoblast and the white yolk.
Of the mode of increase of the epiblast I have but little to say. The cells undoubtedly increase entirely by division, and the new material is most probably derived directly from the white yolk. ”

“ The nuclei are more numerous than before. In the thorax the yolk is anteriorly divided into two lobes by the vertical septum, which contains the vertical muscle of the suctorial pouch. In the posterior part of the thorax it is undivided.
I have not yet been able clearly to make out the eventual fate of the yolk. At a subsequent stage, when the cavity of the abdomen is cut up into a series of compartments by the growth of the septa, described above, the yolk fills these compartments, and there is undoubtedly a proliferation of yolk cells round the walls of these compartments. ”

“ The cells of the latter hemisphere divide more rapidly, and form the future aboral surface. When the dorsal hemisphere has twenty-two cells the ventral has fifty-four. The gastrulation is a combination of epiboly and invagination. The ventral ectoderm grows over, so as to envelop the dorsal hemisphere, while the latter sinks down and becomes saucer-shaped. In the centre of the dorsal surface ten cells form the future endoderm. ”

“ The first form tends to develop into new structure, the second to disintegrate and decay. The tendency to one or other form depends largely on the strength or weakness of the system’s health, on the deficiency or excess of corpuscles in the exuded fluid, and on the distance of the latter from living tissues and blood supply. Much also depends on the predisposition of the genus, the tendency to suppuration in lymph being in a descending series from horse, ass, and mule, through ox and sheep, to dog, pig, and finally, the bird, in which latter suppuration is quite exceptional. ”

“ The third is the so-called avicularium, or bird’s head, also vividly described by Gosse: “The cells [of this particular species] are oblong-shaped, and look much like a sack of corn. Just below one of the spines that crown the summit of the cell on one of the edges is situated a small lump which bears a remarkable resemblance to the head of a bird. It has a strongly hooked beak with two well-formed mandibles, one of which is removable. You observe it deliberately opening, like the beak of a bird and then closing with a strong, sudden snap. The birds' heads are not inhabitants of the cells. ”

“ The abdomen, besides various other parts, contains the honey-bag, the venom-bag, and the anus, which latter in the female comprehends the ovipositor and sting: in the male it contains the organs of reproduction but no sting, and of course no ovipositor. ”

“ The importance of the white yolk is that it passes insensibly into a distinct layer of the germinal membrane, between the two primary layers.127 Each of the three layers of the germinal membrane has its specific character assigned to it by embryologists, who, however, are not all in agreement. Some authorities regard the topmost layer as the origin of the nervous system, the epidermis, with hair, feathers, nails, horns, the cornea and lens of the eye, etc. ”

“ If you turn to the production of36 the chick in ovo, however, you will find at first no more than a vesicle or auricle, or pulsating drop of blood; it is only by and by, when the development has made some progress, that the heart is fashioned: even so in certain animals not destined to attain to the highest perfection in their organization, such as bees, wasps, snails, shrimps, crayfish, &c., we only find a certain pulsating vesicle, like a sort of red or white palpitating point, as the beginning or principle of their life. ”

“ A male, shot near Grande Terre, in the Gulf of Mexico, examined. The skin is very thin, but the subcutaneous cellular tissue is extremely developed, forming a thick reticular layer over the whole body. The internal cells are also of vast size, the right hepatic being 4 1/2 inches long, the right abdominal 4 1/2 by 4; the left abdominal 5 1/2 by 4; the clavicular cell is not formed by a single cavity, but of numberless cellules, like those of the subcutaneous tissue. The heart n is triangular, pointed, 3 inches long, 2 inches and 10 twelfths in breadth ”

“ An excretory system has not hitherto been seen. Yolk-glands are absent. An otolith underlies the brain. The Acoela are marine.
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(2) Rhabdocoela, which possess a complete alimentary tract separated from the body-wall (except for a few suspensory strands) by a space or body-cavity, filled with fluid. This space is sometimes (Vortex viridis) lined by an endothelium of flattened parenchymatous cells. There are two compact testes, which are enclosed (as are the ovaries and yolk-glands) in a distinct membrane. An otolith is present in some genera and species. Terrestrial, fresh-water, marine. ”

“ In the Mammalia we find discontinuity as a general rule. But in quite a number of forms folds of the lining membrane of the abdominal cavity are developed, which practically ensure the passage of the ova into the oviduct when they are extruded from the ovaries. The oviduct, moreover, has a large and fimbriated mouth, called in human anatomy—which is provided with a number of fanciful names—the morsus diaboli. ”

“ The ridge, of which the limbs are special developments, is situated on a level slightly ventral to that of the dorsal aorta, and extends from just behind the head to the level of the anus. It is not noticeable in surface views, but appears in sections as a portion of the epiblast where the cells are more columnar than elsewhere; precisely resembling in this respect the forward continuation of the dorsal fin. ”