1890.] Zoölogy. 679 
recalling to a slight degree Metschnikoff’s,? Pl. XIV., Fig. 5. The 
result of this yolk segmentation is to divide the egg into a number of 
yolk cells, in the center of each of which there is a nucleus with its 
thin layer of protoplasm. 
(2) The result of migration of the products of egg and nuclear seg- 
mentation is the formation of a blastoderm at first on one side of the 
egg, the cells of which are smaller and less charged with yolk than 
those of the rest of the ovum. At this time surface views show no 
traces of regularity. At one pole are numbers of poorly-defined small 
cells, while at the other the cells are greatly larger and fewer in 
number. The blastoderm thus formed produces a lighter spot on one 
side of the egg, which strikingly resembles the primitive cumulus of the 
Arachnids, With the formation of this blastoderm the secretion of 
the blastodermhaut (amnion of Packard, deutovum of my former 
paper?) begins 
(3) In from ‘Gah to eleven days after impregnation (the period 
varies in eggs of the same lot) a small circular pit appears in the center 
of the primitive cumulus. This I regard as the blastopore. . This soon 
becomes triangular and then elongates, while on the next day a 
second cloud appears behind the first, but connected with it. At first 
the second cloud is smaller, but it rapidly attainse quality with the prim- 
itive cumulus, and soon surpasses it. During this process the outlines 
become indistinct, more so than in Balfour’s* Pl. XIX., Fig. 1, 
which in other respects, except in length, agrees well. During this 
process the blastopore increases in length backwards, in the shape of a 
shallow groove (primitive groove), the enlarged anterior end of which 
continues to mark the original site of the first appearance of the 
structure. This primitive groove runs back into the posterior cloud 
and fades out behind. A second lighter area hasnow become promi- 
nent along the margins of the blastopore and its posterior continua- 
tion, produced by the proliferation, as shown by sections, of meso- 
dermal cells from the margins. These wander in between the rest of 
the blastoderm (ectoderm) and the yolk (entoderm) cells which occupy 
the interior. Gastrulation produces no entoderm. 
(4) In fifteen days this primitive groove has become less distinct; 
through the flattening of its walls; while the germinal area, now out- 
lined by the limits of the extension of the mesoderm, has become di- 
vided by the appearance of a transverse groove into cephalic and post- 
? Zeitschr. f. wiss. Zool., XXI, 1871. 
3 Quarterly four. Micros. Sci., XKV., 1885. 
* Q. 7. M. S., XX., 1880. 
Am. Nat.—July.—6. 
