Excerpt from Quarterly Journal of Microscopical Science, 1909, Vol. 53: With Lithographic Plates and d104-Figures

We will in this chapter have to consider the earliest processes by which the cell-material consequent upon the cleavage of the egg comes to be arranged in the fundamental cell-layers out of which the different organs Of the adult animal will gradually take their origin.

And we must in the first place call attention to numerous and important investigations that have taken place, more particularly concerning invertebrate animals, in which the cleavage-cells were followed as far as possible up to their final destination with respect to organogenesis (wilson, '92, '97; Conklin, '97; Casteel, These researches concerning the cell-lineage, as it has been called, have been carried on by the aid Of such worms and molluscs that had eggs as transparent as possible, and, notwithstanding the evident high importance Of the results Obtained, there is for the present little chance of success for similar attempts with the opaque and yolk-laden or deeply hidden eggs Of the Vertebrates. I mention this in order to point out that several questions in dispute might in this way be settled, and that more especially the mammalian egg with its holoblastic cleavage would here offer a most desirable Object Of study. There has been a tendency to suppose that the two primary cell-layers which are encountered in all vertebrate and invertebrate animals, the ectoderm and the endoderm, already become separated from each other when the two first cleavage cells arise.

Others have concluded that in this separation Of the egg cell into the two first cleavage cells the embryonic material was separated into the mother-cells Of the right and the left half of the body or into the anterior and posterior half, as chance would have it (roux). Experiments have even been carried out to prove this. At the present moment we are not in a position to say whether there is any general rule in this respect applicable to all vertebrates, and yet there seems to be hardly any doubt that both in Amphioxus and in man the two opposite extremes in the phylum of the Chordata the two first cleavage cells, if separated from each other.

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