1896.] Embryology. 769 
extend quite to the level of the nuclei, the position of the latter in the 
figure having been taken from subsequent sections in the series. This 
mass, however, is not to be regarded as “debris,” but its presence is 
due to a definite cause to be explained later. The remainder of the 
section is occupied by homogeneous protoplasm filled with rather small 
yolk granules, and surrounded by the thin, transparent vitelline mem- 
brane. 
Under a high power (Fig. 40) the wrinkles appear as deep sinuses 
extending obliquely into the protoplasm, and bordered by a thick layer 
of pigment. These sinuses are angular and irregular in outline, and 
often cantract at their inner ends into long, narrow slits, with rather 
distinct walls. We can now see what it was impossible to detect from 
a surface view, namely, that the wrinkles are compound. 
The larger, principal ones have secondary, smaller ones extending 
outward from their sides, approximately at right angles. Schultze 
observed and figured these compound wrinkles in his surface views of 
R. temporaria, and adds another detail which I have been unable to 
find in the Chorophilus eggs, viz., the breaking-up of a single wrinkle 
at its peripheral end into several smaller ones arranged radially from a 
‘common point. 
The pigment usually fills the projecting protoplasm between adjacent 
sinuses. It is also much thicker in the region of the wrinkles than else- 
where along the furrow, as can be seen in fig. 40. 
In view of these different facts, therefore, it seems evident that there 
is an intimate relation between the wrinkles and the pigment—and that 
both may be results of the same cause. It remains to ascertain what 
this cause is, if possible. 
According to Schultze the egg is a single cell, and just as cellular 
division is brought about by the contractility of protoplasm, so is the 
segmentation of the egg due to the same cause. These contractions 
originate at the point where the furrow begins, and are at first confined 
to a very small area. Since the cortical portion of the egg protoplasm 
possesses a glutinous consistency, it is not to be wondered at that folds 
or wrinkles appear at the same time with the furrow, in its immediate 
vicinity, and at right angles to it. These subsequently disappear in 
consequence of the difference in contractility between the outer and 
inner protoplasm, due to their different consistency. 
It is exceedingly difficult to understand how compound wrinkles, of 
such a nature as we have just described, could be produced by the 
simple contraction of a viscous cortical layer of protoplasm, especially 
if that contraction starts from a fixed point in the layer. Indeed, how 
