Embryo sac

“ As the embryo develops it may absorb all the food material available, and store, either in its cotyledons or in its hypocotyl, what is not immediately required for growth, as reserve-food for use in germination, and by so doing it increases in size until it may fill entirely the embryo-sac; or its absorptive power at this stage may be limited to what is necessary for growth and it remains of relatively small size, occupying but a small area of the embryo-sac, which is otherwise filled with endosperm in which the reserve-food is stored. ”

“ When it reaches the ovary, the sperm cell penetrates an ovule by making its way through a little hole called the micropyle. It then grows toward a clear bit of protoplasm known as the embryo sac. The embryo sac is an ovoid space, microscopic in size, filled with semifluid protoplasm containing several nuclei. (See Figure.) One of the nuclei, with the protoplasm immediately surrounding it, is called the egg cell. It is this cell that the sperm nucleus of the pollen tube grows toward; ultimately the sperm nucleus reaches the egg nucleus and unites with it. ”

“ The sheath is simply the egg membrane which from being oval at first becomes stretched by the developing embryo to finally become a long, narrow sac encasing the fully developed embryo as it exists in man. From the lymph stream they reach the general circulation. In a case of a man with filarial embryos in his peripheral circulation, who committed suicide one morning, Manson found the embryos, in large part, contained in the vessels of the lungs. There were 675 embryos per slide in blood from the lungs for one from blood from the spleen or liver. ”

“ The three embryonic layers are indicated by broken lines, dots, and black lines, just as they are in the frog diagrams. Not only is the embryo slowly pinched off from the yolk sac (y.s.) , but, as the yolk is absorbed beneath it, and it grows in size, it sinks into the space thus made, the extra-embryonic somatopleur and epiblast rise up round it as two folds, which are seen closing in 5, and closed in 6, over the dorsal side of the young chick. In this way a cavity, a., lined by epiblast, and called the amniotic cavity, is formed. ”

“ Parthenogenesis, or the development of an embryo from an egg-cell without the latter having been fertilized, has been described in species of Thalictrum, Antennaria and Alchemilla. Polyembryony is generally associated with the development of cells other than the egg-cell. Thus in Erythronium and Limnocharis the fertilized egg may form a mass of tissue on which several embryos are produced. Isolated cases show that any of the cells within the embryo-sac may exceptionally form an embryo, e.g. ”

“ In reference to his observations he says that, in the unincubated egg, he saw a small embryo enclosed in a sac which he subjected to the rays of the sun. "Frequently I opened the sac with the point of a needle so that the 'animals contained within might be brought to the light, nevertheless to no purpose: for the individuals were so jelly-like and so very small that they were lacerated by a light stroke. Therefore it is right to confess that the beginnings of the chick preexist in the egg and have reached a higher development in no other way than in the eggs of plants." ”

“ Two other lamina—laminæ ventrales of Von Baer—are in the mean time proceeding from the axis of the embryo, one on each side; they grow laterally and converge below the axis.
After the rudiments of organic life have been commenced in the central portion of the serous layer, a fold of its peripheral portion arches over the dorsal surface of the embryo so as to represent a sac, and is composed of two membranes; the one next to the fœtus is the amnion, the other is gradually separated from the amnion and joins the serous lamina of the blastoderma; this is the false amnion of Pander. ”

“ In fact, it is the primitive alimentary apparatus or archenteron, and its wall is termed the hypoblast. The rest of the blastoderm, on the contrary, is the primitive epidermis, and receives the name of epiblast. If the food-yelk were away, and the archenteron enlarged until the hypoblast came in contact with the epiblast, the entire body would be a double-walled sac, containing an alimentary cavity, with a single external aperture. This is the gastrula condition of the embryo; and some animals, such as the common fresh-water polype, are little more than permanent gastrulæ. ”

“ The larger the embryo is in the placentals, and the longer it remains in the womb, the more necessary it is to have special structures to meet its great consumption of food.
In this respect there is a very conspicuous difference between the lower and higher mammals. In the Marsupials, in which the embryo is only a comparatively short time in the womb and is born in a very immature condition, the vascular arrangements in the yelk-sac and the allantois suffice for its nutrition, as we find them in the Monotremes, birds, and reptiles. ”

“ The latter give rise to a closed enteric sack, and probably also to a mass of cells situated on the ventral side, which appear to be mesoblastic. The epiblast is constituted of a single layer of cells which completely envelopes the enteric sack and the mesoblast.
While the ovum is still within the chorion five peculiar processes of epiblast grow out; four of which usually lie in the same sectional plane of the embryo. They are contractile and contain prolongations of the body cavity. Their relative size is very variable. ”

“ But the whole,—Shark, yelk-bag, and all—is imprisoned in a brown horny capsule, that looks like a pillow-case, with long tapes appended to the four corners.
This very peculiar protecting capsule points clearly to a peculiar structure in the parent. The embryo was not inclosed in the pillow-case, at its first formation; but, in the course of its descent from the ovary through the oviduct, it had to pass a region of the latter, where was a thick glandular [Pg 327] mass,—the nidamental gland,—whose office it was to secrete a dense layer of albumen, with which, the embryo became invested. ”

“ After the passing away of the yolk sack, the embryo is nourished and sustained by the "allantois," another peculiar sack which is formed. This sack readily becomes filled with blood-vessels, and serves to nourish the embryo by sustenance obtained from the body of the mother through the walls of the Uterus, a direct communication with the blood-vessels of the mother thus being secured. The blood in the embryo, and that in the mother, come into close contact, thus allowing the embryo to be nourished by the blood of the mother. ”

“ The embryonic development, after irregular segmentation of the ovum into a number of blastomeres, leads to the formation of a solid blastosphere or morula, which is surrounded by a cellular investing membrane (yolk envelope) , while the principal mass of the cells forms the embryo, which uses for its nourishment the yolk cells, which have in the meantime disintegrated (cf. footnote, p. 223) . Usually, after the ova have reached water the embryos hatch out, leaving the yolk envelope in the egg-shell ”

“ The latter succeeds immediately to the former; beginning at the moment when the new being is grafted on the maternal womb, and continuing until its expulsion from thence at the full period of gestation. It follows, therefore, that the Embryonic or former state is that in which the new being is as yet, without any direct or indirect communication with the mother, and still less so with external objects. This state persists for about two weeks after fecundation, during which the Embryo continues to derive its nourishment from the cortical membrane of the Ovum. (Boer, Soemmering, Plagge, myself.) ”

“ The notochord does not long remain attached to the hypoblast, and the separation between the two is already effected for the greater part of the length of the embryo by the stage represented in fig. 129. Fig. 126 represents a series of sections through this embryo.
Fig. 127. Diagrammatic longitudinal section of an embryo of Lacerta.
pp. body cavity; am. amnion; ne. neurenteric canal; ch. notochord; hy. hypoblast; ep. epiblast of the medullary plate; pr. primitive streak. In the primitive streak all the layers are partially fused. ”