1889.] Segmentation of the Ovum. 757 
of deutoplasm, which in the course of evolution has been lost, so 
that in the ova of the PlacentaHa there is very little yolk 
material. We know, further, that the readiness of cellular 
divisions depends on the amount of yolk ; hence when the yolk 
is lost, we should expect to find the entoderm, which, as we have 
seen, is derived from the vegetative substance of the ovum, to be 
represented by relatively small cells. If we imagine the number 
of entodermic cells in the frog's ovum ( Fig. 4, Yolk) reduced, 
their connection with the primitive blastoderm and their char- 
acter as a continuous layer being preserved, we obtain at once 
the characteristic arrangement of the mammalian blastodermic 
vesicle, (Fig. 14.) The homology here established is further 
confirmed by the coarse net-work of protoplasm in the cells of 
the outer layer of the vesicle (Ed. van Beneden, 16), suggesting 
at once the meshes which have been emptied of their deutoplasm. 
Adam Sedgwick, ^j, has shown that in the ova of Peripatiis 
capcnsis the yolk matter has been lost, though abundant in other 
species of the same genus, and the coarseness of the protoplasmic 
net-work is preserved as evidence of the granules formerly 
present. This observation serves to confirm the view I have 
suggested as to the significance of the wide-meshed reticulum of 
the cells of the mammalian sub-zonal layer, (Fig. 14, Yolk) 
The disposition of the animal pole in the ovum before segmen- 
tation also conforms to the homologies here advocated. It will 
be remembered that the protoplasm of the animal pole extends 
far into the ovum, and is enveloped by a cup (deutoplasm zone) 
of the substance of the vegetable pole. Hence when the animal 
pole forms cells they lie as an inner mass, (Fig. 1 1 , i.m) 
If Minot's view be adopted, then the ectoderm lies within the 
entoderm at a certain stage of development, for the one cell 
which retains, as shown in Fig. 8, the connection of the ectoderm 
with the exterior, is subsequently overgrown by the outer layer 
of cells (van Beneden, Heape). There is then a complete inver- 
sion of the germ layers in all (?) placental Mammalia. In most 
cases the inversion is temporary ; the inner mass, as described 
above, flattens out, and probably flattens out inside the outer 
epithelial layer ; if this is the case, then the external layer of the 
