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 cleav- 

 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. 



3. 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 arc, and is a necessary consequence of the infolding of the surface 

 layer to form the groove. 



