1896.] 



Embryology. 



771 



make up for the shortening of the arc by protruding a little from the 

 surface of the egg at either end of the furrow. Such a condition is 

 admirably shown when one creases the top of a soft felt hat, and would 

 necessarily be even more manifest if the hat were filled with a viscous 

 fluid. 



An examination of any of the surface views will show that this i& 

 not the case with these Chorophilus eggs, but that the first indication of 

 the groove at either extremity is a slight hollowing in of the surface, 

 and not a bulging outward. This view is confirmed by a study of the 

 sections. 



The other alternative is that the shortening of the arc must result in 

 longitudinal condensation along the bottom of the furrow, starting at 

 the center and increasing as the furrow progresses. 



Such a shortening or contraction along the bottom of the furrow 

 would very naturally throw its sides into folds or wrinkles at right 

 angles to its length. The pigment layer in contact with these walls 

 would also be thickened in the region of the wrinkles, and toward the 

 center of the groove. 



Since the condensation starts at the center and advances in both 

 directions with the progress of the furrow, the wrinkles would be ar- 

 ranged somewhat radially about the superior pole. This progressive 

 contraction also accounts for the successive appearance and disappear- 

 ance of wrinkles, and for the confluence of smaller into larger ones. 



As the sides of the furrow begin to fuse into the permanent segmenta- 

 tion plane or cell-wall the wrinkles disappear through the gradual re- 

 adjustment of former relations. 



Indeed, the whole phenomenon seems very largely dependent on the 

 rapidity of segmentation and the consequent sudden disturbance of 

 normal relations before the diflferent portions can adjust themselves to 

 their new conditions. 



This fact will serve to explain why the wrinkles show so prominently 

 in this particular species, which has a very rapid development, and also 

 why the conditions under which they were examined — the transference 

 from ice to tepid water and the placing of the eggs in strong sunlight — 

 were especially favorable. 



It may also suggest a reason why one observer has failed to detect 

 wrinkles in the eggs of a given species, while another, working under 

 more favorable conditions, has seen and described them. And it will 

 in a measure account lor the absence of wrinkles on the yolk hemisphere, 

 smce segmentation is very much slower there. 



