4 A. A. W. HUBRECHT. 
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, 792, 
97; Conklin, 797; Casteel, 704), ; 
These researches concerning the “ cell-lineage,” as it has 
been called, have been carried on by the aid of such worms 
and molluses 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 eges 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 celi-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, 
