1896.] 



Embryology. 



765 



most careful scrutiny fails to detect the actual motion, or to see evi- 

 dences of any movement of the protoplasm within the egg, which might 

 cause the wrinkling. 



The furrow at the pole has become quite deep by the time its ends 

 have reached the equator, in five or six minutes. The ends seem to stop 

 here for a time, just at the border of the yolk area, while further changes 

 take place in the pigmented portion. First the two edges of the groove 

 approach each other at the pole, and seem to fuse slowly, the wrinkles 

 entirely disappearing during the process. This fusion then extends in 

 either direction along the groove for some distance, often half way to 

 the equator, obliterating the wrinkles as it goes. By this means the 

 groove may entirely disappear at the center while remaining near the 

 periphery (Fig. 38). 



At this point the furrow begins to enter the yolk area on either side, 

 and at the same time the groove reappears along the center of the pig- 

 mented hemisphere. It now becomes very broad and deep. Indeed, it 

 seems to reach clear through to the yolk, and its walls are considerably 

 rounded on either side. But they are now smooth, so that the wrinkles 

 remain in all fifteen or twenty minutes on the first furrow. 



This first furrow divides the egg into two nearly equal parts (Figs. 

 3, 12, 28, 35). When its ends first reach the yolk area, where they 

 stop for a time, as already noted, the two blastomeres thus formed are 

 very much rounded at their ends, and diverge strongly from the groove. 

 This is readily seen in the series given in figs. 1 to 4, but it becomes 

 much more prominent after the reappearance of the groove — (Figs. 28 

 to 30). Under a higher magnifying power a wide space can now be 

 seen at either end between the two segments. The floor of this space is 

 triangular in shape and doubly curved, being concave from side to 

 side, and convex antero-posteriorly. It is formed of light colored yolk, 

 into which the pigment shades gradually around the borders. The two 

 segments are thus rounded in a manner very similar to that of the first 

 two blastomeres of a meroblastic egg. 



As the furrow proceeds toward the inferior pole the space between 

 the two pigmented segments diminishes, the borders of the furrow 

 approach each other, and the surface of the egg becomes smooth once 

 more, with the groove indicated merely by a narrow, faint line. 



It remains in this condition some eight or ten minutes before the 

 second cleavage begins, and this may be called its resting stage. The 

 rapid closing of the groove previous to the appearance of the second 

 furrow occurs also in Amblystoma (Eycleshymer). 



