The two plates of mesoblast are at first unconnected with any other cells of the blastoderm, and, on their formation, the hypoblast remains in connection with all the remaining lower layer cells. Between the embryonic rim and the yolk is a cavity,—the primitive alimentary cavity. Its roof is formed of hypoblast, and its floor of yolk. Its external opening is homologous with the anus of Rusconi, of Amphioxus and the Amphibians. The ventral wall of the alimentary cavity is eventually derived from cells formed in the yolk around the nuclei which are there present.
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Since the important researches of Gegenbaur[149] upon the meroblastic vertebrate eggs, it has been generally admitted that the ovum of every vertebrate, however complicated may be its apparent constitution, is nevertheless to be regarded as a simple cell. This view is, indeed, opposed by His[150] and to a very modified extent by Waldeyer[151], and has recently been attacked from an entirely new standpoint by Götte[152]; but, to my mind, the objections of these authors do not upset the well founded conclusions of previous observations.
As soon as the fact is recognised that both meroblastic and holoblastic eggs have the same fundamental constitution, the admission follows, naturally, though not necessarily, that the eggs belonging to these two classes differ solely in degree, not only as regards their constitution, but also as regards the manner in which they become respectively converted into the embryo. As might have been anticipated, this view has gained a wide acceptance.
Amongst the observations, which have given a strong objective support to this view, may be mentioned those of Professor Lankester upon the development of Cephalopoda[153], and of Dr Götte[154] upon the development of the Hen's egg. In Loligo Professor Lankester shewed that there appeared, in the part of the egg usually considered as food-yolk, a number of bodies, which eventually developed a nucleus and became cells, and that these cells entered into the blastoderm. These observations demonstrate that in the eggs of Loligo the so-called food-yolk is merely equivalent to a part of the egg which in other cases undergoes segmentation.
The observations of Dr Götte have a similar bearing. He made out that in the eggs of the Hen no sharp line is to be found separating the germinal disc from the yolk, and that, independently of the normal segmentation, a number of cells are derived from that part of the egg hitherto regarded as exclusively food-yolk. This view of the nature of the food-yolk was also advanced in my preliminary account of the development of Elasmobranchii[155], and it is now my intention to put forward the positive evidence in favour of this view, which is supplied from a knowledge of the phenomena of the development of the Elasmobranch ovum; and then to discuss how far the facts of the growth of the blastoderm in Elasmobranchii accord with the view that their large food-yolk is exactly equivalent to part of the ovum, which in Amphibians undergoes segmentation, rather than some fresh addition, which has no equivalent in the Amphibian or other holoblastic ovum.
Taking for granted that the ripe ovum is a single cell, the question arises whether in the case of meroblastic ova the cell is not constituted of two parts completely separated from one another.
Is the meroblastic ovum, before or after impregnation, composed of a germinal disc in which all the protoplasm of the cell is aggregated, and of a food-yolk in which no protoplasm is present? or is the protoplasm present throughout, being simply more concentrated at the germinal pole than elsewhere? If the former alternative is accepted, we must suppose that the mass of food-yolk is a something added which is not present in holoblastic ova. If the latter alternative is accepted, it may then be maintained that holoblastic and meroblastic ova are constituted in the same way and differ only in the proportions of their constituents.
My own observations in conjunction with the specially interesting observations of Dr Schultz[156] justify the view which regards the protoplasm as present throughout the whole ovum, and not confined to the germinal disc. Our observations shew that a fine protoplasmic network, with ramifications extending throughout the whole yolk, is present both before and after impregnation.
The presence of this network is, in itself, only sufficient to prove that the yolk may be equivalent to part of a holoblastic ovum; to demonstrate that it is so requires something more, and this link in the chain of evidence is supplied by the nuclei of the yolk, which have been so often referred to.