772 The American Naturalist. [September, 
If our explanation is a correct one there ought to be no wrinkles at 
all along the bottom of the groove, while they should be present and 
have their greatest depth about half way between the bottom and the 
surface. 
The sections show that this actually occurs. Fig. 39 is a section cut 
just at the level of the bottom of the groove, and shows no trace of any 
wrinkles, nor are there any in the two or three preceding sections. 
They then appear and gradually increase in size up to the level of 
fig. 40, which is a magnified portion of the same groove about half way 
to the surface. 
The problem of the compound nature of the wrinkles finds its solu- 
tion in the fact that there must be a condensation along the bottom of 
the larger wrinkles, in all respects similar to that in the groove, and 
due to the same cause, though, of course, on a very much smaller scale. 
But in this instance the condensation would proceed in only one 
direction, and hence we find the secondary wrinkles all inclined in the 
same direction to the principal ones, just as we have already observed, 
and as Schultze has so finely figured. 
Summary.—1. Subjection to a temperature of 0° C. for a period of 
eight hours completely arrests all development for the time being, but 
results, on the subsequent restoration of ordinary conditions, in a cleay- 
age more rapid than that of normal eggs. 
2. Segmentation, at least up to the 128-cell stage, is accompanied by 
the formation, fusion and subsequent elimination of well defined wrinkles 
along the sides of the furrows in the pigmented area. There are no 
wrinkles on the yolk, except along the inferior border of the third 
cleavage furrow. 
As seen in an examination of cross-sections these wrinkles are 
compound in nature, the larger, principal ones having smaller second- 
ary ones along their sides. 
4, The wrinkles on the first furrow are arranged somewhat radially 
about the superior pole. On subsequent furrows they are inclined at 
an angle toward the point where the furrow starts. 
5. The pigment which borders the segmentation furrows forms a 
thicker layer in the region of the wrinkles than elsewhere along the 
groove, thus showing an intimate relation between the two. 
6. The probable cause of the wrinkling is to be found in the condensa- 
tion along the bottom of the groove, which results from the shortening 
of the are, and is a necessary consequence of the infolding of the surface 
iyor to form the groove.. 
