47 S The American Naturalist. [June, 
pletely and produces several layers of cells (the uppermost 
becoming epithelioid), which are the ectoderm or primitive 
blastoderm ; the edge of the blastoderm touches the yolk and 
is surrounded by a nucleated zone in which the production of 
cells is continuing ; underneath the blastoderm is the fissure- 
like segmentation cavity ; the floor of this cavity is formed 
by the unsegmentated yolk (entoderm) which is furnished 
with scattered nuclei in the layer immediately under the seg- 
mentation cavity ; the yolk nuclei, at least in selachians and 
reptiles, are of two kinds, very large ones and smaller ones, 
which arise, probably, from the large nuclei ; the nucleated 
layer may be termed the sub-germinal plate. 
Modified segmentation of placental mammals. The lowest mam- 
mals resemble the reptiles in many respects ; amongst other 
reptilian characteristics of the monotremes, we find ova of 
large size and rich in deutoplasm. That these ova segment 
during their passage through the oviduct, in similar manner 
to those of reptiles, was first ascertained by direct observa- 
tion by Caldwell in 1884, 16. 
In marsupials and the placental mammalia the amount of 
yolk substance is greatly reduced and the ovum is of small 
size. It is, therefore, holoblastic, that is to say, the cleavage 
planes cut through the entire cell, as in the primitive type of 
segmentation, but the arrange- 
ment of the cells at the close of 
segmentation appears to be a 
direct inheritance from the rep- 
tilian ancestors of the mammals. 
The segmentation of the mam- 
malian ovum was first clearly re- | 
cognized by Bischoff, though it 
had been previously seen and 
misinterpreted by Barry, 5, 6, 7 j 
very beautiful figures of segmen- 
tation in the rabbit have been pj^ ^ Ovum of a rabbit of twenty- 
given by Coste, 17. More re- four hours; after Coste. The 
^, , ^. , , first cleavage has been completea, 
Cently, observations have been the two cells are appressed; above 
published by Hensen on the rab- ^^^4;=^^^^^^^^^ erma?oza' lie tn'' ani 
bit, 24, van Beneden on the rab- within the zona pellucida. 
