EMBRYOLOGY OF CRYPTOBRANCHUS 141 



prominent. This phenomenon is much more marked in some 

 other cases than in the one figured. 



In eggs taken about three hours after fertihzation (see fig. 42), 

 the thickening of the cell wall has flattened to the form of a disc; 

 it is strongly striated, recalling the zona radiata from which it 

 takes its ultimate origin. The perforation made by the spermato- 

 zoon has disappeared. The cytoplasmic path of the spermato- 

 zoon has become filled with yolk, except for a broad shallow region 

 underlying the thickening of the cell wall. The head of the sper- 

 matozoon has become shorter and thicker, and takes brilliantly 

 the nuclear stain; the tail has become slightly shorter, perhaps 

 by the degeneration of the posterior portion. The radiations of 

 cytoplasm proceeding from the region of the middle-piece have 

 disappeared, but in the same locality there is a somewhat larger 

 spherical region of uniformly distributed yolk-free cytoplasm. 



Five hours after fertilization (see fig. 43), the spermatozoon 

 has penetrated only a little deeper into the egg. The thickening 

 of the cell wall of the egg at the place of entrance of the spermato- 

 zoon has disappeared, but its site is marked by convolutions in 

 the cell wall. The protoplasmic path leading from the surface of 

 the egg to the spermatozoon has almost entirely disappeared, but 

 traces of it persist at intervals along the route. The head of the 

 spermatozoon is spindle-shaped and much shorter and thicker 

 than before; the tail persists, but is somewhat abbreviated. The 

 circular area of cytoplasm surrounding the head of the spermato- 

 zoon has expanded to form a large crescent, whose horns extend 

 nearly at right angles to the path of the spermatozoon. The 

 yolk granules underlying the crescent are decidedly coarser than 

 those above it. This suggests a correlation of the internal struc- 

 ture with the 'sperm area' seen from the surface: the horns of 

 the crescent produced would meet the margin of the sperm area 

 (compare figs. 9 to 11 with figs 43 to 45). 



Seven and one-half hours after fertilization (see fig. 44) the pro- 

 toplasmic path is marked only by a region of sparsely distributed 

 yolk granules extending from the surface for about two-thirds of 

 the distance to the spermatozoon. The yolk granules are par- 

 ticularly dense in the region immediately above the crescent, and 



