1896.] 



Embryology. 



767 



32-cell stage. Other authors, with possibly a single exception, make 

 no mention of them, except along the first furrow. 



This confining of the wrinkles almost exclusively to the pigmented 

 area is manifestly connected with the different organization of the two 

 halves of the egg. The pigmented half is richer in protoplasm, and is 

 to a higher degree under the influence of the cell nucleus ; while the 

 yolk has its protoplasm scattered about amongst the yolk granules, and 

 is also further removed from the nucleus which lies in the pigmented 

 half in the undivided egg (Hertwig). This results in the more rapid 

 segmentation of the pigmented cells, and the presence of the wrinkles 

 seems intimately associated with this rapidity of segmentation. 



The Fourth Cleavage. — This appears from fifteen to twenty minutes 

 after the third. 



In this cleavage also the furrows started in every instance from the 

 'periphery of the four superior quadrants and move toward the pole, 

 accompanied by wrinkles. These latter are now much smaller than 

 heretofore, and are not easily detected under a low power. They are 

 also very transitory, and disappear almost immediately. There is some- 

 what of a tendency in these furrows to run nearly parallel to the first 

 or second vertical, recalling the conditions in teleosts (Figs. 26, 27). 



There is a subsequent fusion and elimination of the furrows after each 

 cleavage, as has already been noted in the first segmentation. 



This elimination of the grooves, due to the fourth cleavage, leaves 

 the pigmented pole of the egg divided into four cells of a totally differ- 

 ent shape and arrangement from that of the four blastomeres resulting 

 from the third cleavage (Fig. 37). After remaining thus for several 

 minutes the furrows reappear, and the cells resume the shape seen in 

 fig. 36. 



From this point segmentation proceeds very rapidly, and in a manner 

 exactly similar to that of other frog's eggs. The wrinkles have now 

 become so small as to be seen only with the greatest diflSculty and 

 under a high power, and they disappear so quickly as to easily escape 

 detection. But they are present at least up to the 128-cell stage, and 

 appear, fuse, and disappear, as in the first cleavage. 



Gastrulation begins about twelve hours after the first cleavage, and 

 the blastopore closes at about the fifteenth hour. The neural folds 

 appear and gradually fuse to form the neural canal as in Rana. By 

 the end of the first day the embryo has elongated considerably, and the 

 head is well differentiated. 



The tail then becomes defined, the gill folds appear, and the eyes are 

 seen as two minute black spots. And by the end of the third day the 

 embryo escapes from the egg envelopes and swims about freely. 



