Gastrula

“ The young amphioxus has, at this stage, which is called the gastrula stage, a curious parallelism with such a lowly form as the Hydra of our ditches. This latter creature, like the gastrula, consists essentially of two layers of cells, an outer protective and sensory layer, and an inner digestive one; it has a primordial intestine, or archenteron, and its mouth is sometimes regarded as being a blastopore. All animals that have little yolk, and start early in life for themselves, pass through a gastrula stage, substantially the same as this of amphioxus. ”

“ As the middle plate curves in and becomes overgrown by the folds forming the edges of the blastopore, the gastrula-tube (Fig. 539, A, r) is formed, and furnishes the rudiments of the lower (inner) layer. The ectoderm, then, according to Heider, arises from the lateral plates of the primitive band. The growth of the edges of the blastopore, by which the closure of the gastrula-tube is effected, takes place latest in the region of the most anterior part of the furrow (Fig. 515, B and C) , corresponding to that place in the primitive band in which the stomodæum afterwards develops. ”

“ You can hardly have failed to notice how closely the gastrula corresponds to a hydra, and many facts lead us to believe that the still earlier ancestor of the hydra was free swimming, and that the tentacles are a later development correlated with its adult sessile life. Yet we must not forget that the hydra is even now not quite sessile, it moves somewhat. And our ancestor was almost certainly a free swimming gastræa, or hypothetical form corresponding in form and structure to the gastrula. The ancestor of man never settled down lazily into a sessile life. ”

“ The blastula, or hollow sphere of segmentation cells, usually ciliated, was reminiscent of the planæa, an ancestral free-swimming form whose nearest living relation is the spherical Magosphæra. The gastrula, finally, is the two-layered sac formed from the blastula, typically by invagination of its wall. It repeats the organisation of the gastræa, which is the common ancestor of all Metazoa, and finds its nearest living counterpart in the simple "sponges" Haliphysema and Gastrophysema. ”

“ In the middle of its hinder border we have the beginning of the invagination of the primitive gut (Figure 1.56 ud) ; it extends gradually from this spot (which corresponds to the Rusconian anus of the amphibia) forward and around, so that the primitive mouth becomes first crescent-shaped and then circular, and, as it opens wider, surrounds the ball of the larger food-yelk.
Essentially different from the wide-mouthed discoid gastrula of most of the selachii is the narrow-mouthed discoid gastrula (or epigastrula) of the amniotes, the reptiles, birds, and monotremes ”

“ This tube communicates with the exterior, in virtue of its mode of formation, by a single opening which is called the "blastopore," which becomes the anus of the later larva and adult. Whilst the gastrula stage is being acquired, the blastocoel or primary body-cavity is invaded by wandering cells budded from the wall of the archenteron (Fig. 282, A, B, C, mes) . ”

“ The type of the Gastræada is the Gastræa, of whose original nature the mouth-and-stomach larva (Gastrula) , which recurs in the most different animal tribes, still gives a faithful representation. (Frontispiece Fig. 5, 6.) Out of the Gastræa, as we have previously mentioned, there were at one time developed two different primary forms, the Protascus and Prothelmis; the former must be looked upon as the primary form of the Zoophytes, the latter as the primary form of Worms. (Compare the enunciation of this hypothesis in my Monograph of the Calcareous Sponges, vol i. p. 464.) ”

“ When it is reduced to a minimum, the Morula is an almost solid aggregation of blastomeres, which may be nearly equal in size, or some may be much larger than others, in consequence of having undergone less rapid division. The next stage in the development of the embryo of a Metazoon consists (in all cases except a few parasitic anenterous forms) in the conversion of the Morula into a body having a digestive cavity, or a Gastrula. The animals in which the embryo takes on the form of a Gastrula, may be termed, as Haeckel has proposed, Gastrcece. ”

“ The Blastula of most animals assumes a new larval form called Gastrula, in which the essential characteristics are that a portion of the blastoderm by invagination converts the Blastula into a cup with double walls, enclosing a new cavity, the primitive gut. ”

“ Gaster is the Greek for stomach. Let us, therefore, call this primitive parent of all the sponges polyps, medusaæ, worms, crustacea, insects, snails, mussels, cephalopods, fishes, salamanders, lizards, birds, mammals, and man, the gastræa, the primitive-stomach 277or primitive-gut animal. The corresponding embryonic form may be distinguished from it as the gastrula. There are still many living species of animals that are very little higher in organisation than the gastræa-form. ”

“ Gastro-coeli: a pair of usually transverse lateral pits near the base of the second abdominal tergite in some Hymenoptera.
Gastro-ileal folds: occur in some insects at the junction of the chylific ventricle with the ileum and serve as a valve.
Gastrula: that embryonic stage resembling a sac, with an outer layer of epiblastic cells and an inner layer of hypoblastic cells. ”

“ We find, first of all, in the lowest Metazoa, the Gastræads, that the gut remains permanently in the very simple form in which we find it transitorily in the palingenetic gastrula of the other animals; it is thus in the Gastremaria ( Pemmatodiscus) , the Physemaria ( Prophysema) , the simplest Sponges ( Olynthus) , the freshwater Polyps ( Hydra) , and the ascula-embryos of many other Cœlenteria (Figs. 233–238) . Even in the simplest forms of the Platodes, the Rhabdocœla (Fig. 240) , the gut is still a simple straight tube, lined with the entoderm ”

“ The archenteron becomes the gastrovascular system or coelenteron. Between the ectoderm and endoderm a gelatinous supporting layer, termed the mesogloea, makes its appearance. The gastrula has now become an actinula, which may be termed the distinctive larva of the Cnidaria, and doubtless represents in a transitory manner the common ancestor of the group. In no case known, however, does the actinula become the adult, sexually mature individual, but always undergoes further modifications, whereby it develops into either a polyp or a medusa. ”

“ In other vertebrates, where more food-yolk is present, the shape and structure of the body render it necessary for the food-yolk to be stored away on the ventral side of the alimentary canal. This, combined with the unsymmetrical position of the anus, which primitively corresponds in position with the blastopore or anus of Rusconi, causes the asymmetry of the gastrula invagination, since it is not possible for the part of the ovum which will become the ventral wall of the alimentary canal, and which is loaded with food-yolk, to be invaginated in the same fashion as the dorsal wall. ”

“ The name Planula signifies "wandering animal," because the Planula larva swims about by means of cilia.
Fig. 6.—Diagrammatic representation of a typical Gastrula, or two-layered larval form, highly magnified; optical section, longitudinal. Ec, Ectoderm or skin layer; En, Endoderm or stomach layer; m, mouth leading into the enteric cavity. The dots are the nuclei of the cells.
Fig. 7.—Diagrammatic representation of a typical Trochosphere, or ciliated larva, considerably magnified. M is the mouth; the stomach and intestine are seen showing through the transparent body. ”

“ Pressure from without, also, while it aggravates most pains dependent on local organic [75] mischief, nearly always more or less relieves gastralgia. Equally striking is the comfort given by stimulants, especially by hot brandy-and-water; in this respect gastralgia resembles colic. There is something special in the degree of mental depression which attends gastralgic pain. In this it resembles the pains of hypochondriasis, but there is a resilience of the spirits when the pain has been relieved which is not seen in the latter affection. ”

“ A thraldom of measureless evil oppressed the air. Gaspard felt that the awful transmigration, evoked and implored with everswelling, liturgic blasphemies, was about to take place—had perhaps already occurred. He thought that the breathing giant stirred, like one who tosses in light slumber. ”

“ Pale, panic-stricken faces milled everywhere in an aimless stream. Hurrying torches flared dolorously in the twilight that deepened as if with the shadow of impending wings arisen from Erebus. The gloom was clogged with intangible fear, with webs of stifling oppression. Through all this rout of wild disorder and frenzy, Gaspard, like a spent but indomitable swimmer breasting some tide of eternal, viscid nightmare, made his way slowly to his attic lodgings. ”

“ Anæmia is a specially frequent attendant of gastralgia, more so than of other neuralgias. Women are, by the general consent of authors, more liable to gastralgia than men.
The special mark of true neuralgic pain in the abdominal pneumogastric, as distinguished from other deep-seated pains in the epigastrium, is the remarkably direct relation of its severity to the patient's exhaustion, particularly in regard to the weakness induced by want of food. ”

“ The dwarf squealed and bit Corsus's thumb to the bone, so that he loosed his hold; and the dwarf fled from the hall, while the company laughed pleasantly.
"So flieth folly before wisdom which is in wine," said the King. "The night is young: bring me botargoes, and caviare and toast. Drink, Prince. The red Thramnian wine that is thick like honey wooeth the soul to divine philosophy. How vain a thing is ambition. This was Gaslark's bane, whose enterprises of such pitch and moment have ended thus, in a kind of nothing. Or what thinkest thou, Gro, thou which art a philosopher?" ”

“ By gastralgia, considered as a distinct affection, however, we mean a purely neuralgic condition of the sensory fibres of the stomach, excluding inflammatory and structural changes on the one hand and chronic forms of atonic dyspepsia on the other. The attacks are usually periodical in character, with constricting pain in the pit of the stomach, and the intervals are not necessarily associated with symptoms of dyspepsia. It chiefly occurs in females of nervous temperament at the catamenial periods. ”

“ They met, poured wine over each other and parted. After another month the need of a further stimulant was felt. They met again, and agreed to insult each other weekly.
On the last day of his training Gaspard spoke seriously to his instructor.
"You see that I make nothing of it," he said. "My thoughts are ever with the stomach that I leave behind. Not once have I been in a position to take control. How then can I fight? ”

“ The hinder end of the gullet is produced into a short, wide, membranous funnel projecting freely into the midgut or stomach. The latter is elongated and oval, composed of four rows of hexagonal cells, with large nuclei. This is separated by a distinct constriction or sphincter from the short pear-shaped rectum, which opens by a minute anus on the back just in front of the pedal processes.
The food is chiefly organic débris; but Gastrotricha have been seen to attack large Infusoria by nibbling, and to swallow the protoplasm as it exudes from the wound in their prey. ”

“ The ventral sucker is concealed, but the transparency of the skin permits a view of the internal organs.
Another remarkable genus, established by Von Siebold, is Gasterostoma. In this genus the ventral sucker has taken the position usually assigned to the oral opening; the latter being near the centre of the body. The digestive cæca also disappear, leaving only a short stomachal cavity, which reminds one of the same viscus in imperfectly organised sporocysts or rediæ. When G. gracilescens first came under my observation I followed Rudolphi in describing it as a distome (D. gracilescens) . ”

“ Still these nervous masses have grown dorsalwards, have commenced to involve the walls of the cephalic stomach even in the lowest vertebrate, as is seen in Ammocœtes, in which animal a ventral portion of the epithelial bag has been evidently compressed and its lumen finally obliterated by the growth of the nerve-masses on each side of it. Throughout the whole vertebrate kingdom this obliterated portion still leaves its mark as the raphé or seam, which is so characteristic of the infra-infundibular portion of the brain. ”

“ The frail, dying body of Nathaire, as if possessed by some sardonic fiend, rocked to and fro on the luxurious couch in a long, violent gust of laughter, without other reply.
"If your looks bear creditable witness," said Gaspard, when the baleful laughter had ceased, "you are mortally ill, and the time is short in which you can hope to atone for your deeds of malice and make your peace with God—if indeed it still be possible for you to make peace. What foul and monstrous brew are you preparing, to ensure the ultimate perdition of your soul?" ”

“ In a case seen by the writer a cancerous growth had invaded the entire stomach, causing an enormous thickening of its walls, yet no tumor was to be felt, and the most marked symptoms were gastralgia and exhaustion.
The relations of gastralgia to the other purely functional disorders of the stomach are interesting and peculiar. It is beyond a question that every variety of digestive disorder, from simply delayed and painful digestion to pyrosis, the formation of gas, and constant vomiting, is much more often of purely neurotic origin than has been supposed. ”