1880. | A Sketch of Comparative Embryology. 247 
and fairly represents a gastrula of Amphioxus. In C, the differ- 
ence is very great, and corresponds to a form observed in certain 
Gasteropods. In D, the inner set is no longer separated into dis- 
tinct cells, although there are a number of nuclei, each of which 
marks the center of a future cell. In such an instance we should 
regard the whole inner set as a nutritive yolk, not yet transformed 
into a definite cell-layer. This figure is particularly instructive, 
because it shows that what we call the yolk is not something dis- 
tinct from the germ, but really belongs to 
the inner layer of the embryo. Æ shows a. 
similar egg, in which the outer set of cells 
has not yet grown around the yolk. This 4 
outer layer was called by the earlier embry- 
ologists the blastoderm, in all those eggs 
with a great deal of yolk. F shows the 
Same egg not in section, but seen from the 
outer surface, to exhibit the cap of small 
cells, or the blastoderm, resting upon the 
large yolk. Those eggs in which the differ- fis i i Formion si 
ence in size between the two sets of cells is the blastoderm in Oniscus 
not excessive (A-C) are called holodlastic, ™¥707s after Bobretzky. 
while those in which the yolk remains more or less intact for a 
considerable time (D-F) are termed meroblastic. 
In order more fully to illustrate the peculiarities of the process 
of segmentation, it is necessary to consider the 
holoblastic eggs further. Fig. 13 represents an 
actual section of an egg of the sow-bug, Oniscus, 
after Bobretzky, corresponding very nearly to the 
diagram Æ, of Fig. 12. Fig. 14 is a similar sec- 
tion through the egg of a moth (Pieris crataegi), 
and shows a number of nuclei, each surrounded 
by a little mass of protoplasm, and scattered 
irregularly through the yolk. Their number 3 
gradually increases, and each one becomes the oa ee 
center of a distinct cell. This is merely a pecu- pon S 14 fee 
liar modification of the ordinary method of cell ing egg of a moth; — 
division into two equal parts, for in the moths and“ neater 8 
butterflies and some other animals the large yolk divides gradually, _ 
P 
o 
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Pre 
ey 
A 
P, 
na 
A 
a 
A a. 9 
CY 
F 
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ey 
Ee 
ee 
by forming several nuclei, and so breaking up into a considerable - ae 
number of cells piled up one over the other. ` We shall have oc- — 
