4/6 The American Naturalist. LJ^ne, 
30, Balfour, 2, Sarasin, 41, Weldon, 49, and Hofmann 
Archives ne'erlandaises xvi, i88i) Hofmann gives a resume) 
in Broun's Thierreich vi. Abth. iii. p. 1877-1881. The process 
is more irregular, and small cells are budded off singly and in 
scattered clusters from the larger segments. At the close of 
segmentation the germinal disc is converted into a membrane 
consisting of several layers of cells and parted from the 
underlying yolk by a thin space — the segmentation cavity ; 
at its edge this membrane, the primitive blastoderm, is united 
with the yolk, it being immediately surrounded by a segmen- 
tating zone, from which it receives accretions. The layer of 
the yolk immediately under the segmentation cavity contains 
scattered nuclei, lying singly or in clusters ; each nucleus is 
surrounded by protoplasm ; the nuclei are not all alike ; 
some are very large round with very distinct nuclear threads; 
others are small and often bizarre in shape ; probably the 
latter are budded off from the former. 
In Elasmobranchs, the germinal disc is thicker, and conse- 
quently the mass of cells resulting from its segmentation cuts 
in quite deeply into the yolk, Balfour, Comp. Embryol. i, fig. 
46, Riickert, 40, 28. As segmentation progresses, the cells 
spread out into a layer, which shows the same essential rela- 
tions as have been described in birds and reptiles. There is 
the several-layered primitive blastoderm with its edges con- 
nected with the yolk and itself overlying the segmentation 
cavity, the lower floor of which is formed by the multinucle- 
ate yolk the representative of the cellular yolk mass of the 
frog (Fig. 4, Yolk). The nuclei are confined to the layer im- 
mediately under the segmentation cavity, and this layer cor- 
responds to the sub-germinal plate in teleost ova. Of the 
yolk-nuclei some are large, others are small as in reptiles ; 
they are the Parablast-kerne of His, the Merocy ten- kerne of 
Ruckert. 
In bony fishes, also, we find the same type, but modified 
somewhat. The process of segmentation has been very care- 
fully studied by C. O. Whitman, l, to whom I am indebted 
for the accompanying semi-diagrammatic figure of the seg- 
mented ovum of a flounder. The ovum is surrounded by a 
vitelline membrane, s, from which it has slightly withdrawn, 
