Segmentation of the Ovum. 
otably J 
: the upper pole, where 
constituting the blastoderm, Bl.\ 
the outer layer of cells is just beg 
lioid character ; under- 
neath the blastoderm 
is the well-marked seg- 
everywhere at the edge 
of the blastoderm lies 
the segmenting zone. 
lar protoplasm with rap- 
idly dividing nuclei 
the cells result 
these divisions 
ed to the edge of the 
blastoderm, which thus 
enlarges peripherally. 
The protoplasm of the 
segmenting zone is pro- 
longed inwards form- 
ing the floor of the seg- 
mentation cavity ; this 
sheet of protoplasm s. g. 
The segmenting zone i 
thick cap of cells 
stage represented, 
gl. oil globule of yolk. 
wn as the sub -germinal plate. 
lurse, the homologue of the 
the so-called germinal wall,, 
but it is quite sharply defined against the yolk and therein 
differs from the wall in the chick, because in the latter the 
germinal merges gradually into the yolk. The process of 
segmentation differs from that in elasmobranchs and saur- 
opsida, in that the cleavage of the germinal disc is strikingly 
regular, and further in that the whole width and thickness 
of the germinal ( 
-ivolved in the segmentation from the 
very start. The segmentation in teleosts is further interest- 
ing as affording proof that all the nuclei as shown by Whit- 
man's investigations, arise from the segmentation nucleus. 
To summarize : In vertebrate ova with a large yolk which 
does not divide into cells until segmentation is considerably 
advanced, the substance of the animal pole segments com- 
