BILATERALITY IN CRYPTOBRANCHUS 379 



and in the seven remaining eggs the axis of excentricity is approxi- 

 mately at right angles to the first cleavage furrow. Evidently 

 there is no constant relation between the two features con- 

 sidered. 



3. Relation to the entrance-path of the spermatozoon. In the 

 experiments already described in which an attempt was made 

 to control the direction of entrance of the spermatozoon, excen- 

 tricity in the cleavage pattern of the early blastula was observed 

 in fifteen eggs. No doubt a much larger number of cases of 

 excentricity would have been found had all the eggs been kept 

 under continuous observation. In these fifteen eggs (fig. 19) 

 there was an evident lack of uniformity in the direction of the 

 axis of excentricity with respect to the probable direction of the 

 entrance-path of the spermatozoon. 



E. The excentricity manifested by the internal structure of the early 



blastula 



In order to study the internal development of Cryptobranchus 

 during the cleavage stages, entire eggs were embedded in paraffi.n 

 and cut into vertical serial sections (i.e., sections taken in planes 

 parallel to the polar axis of the egg). Of these sections the ones 

 passing approximately through the center of the egg are desig- 

 nated as meridional. In a large number of cases the eggs were 

 not oriented with reference to any structural features other 

 than polarity. In examining sections of such eggs in stages 5 to 

 9, inclusive (early blastula to very late blastula), the writer was 

 quickly impressed with the fact that the internal development of 

 the micromeres is excentric with respect to the animal pole. On 

 one side of the axis of polarity the micromeres composing the roof, 

 and especially the lateral wall, of the blastocoele are smaller and 

 more numerous, richer in cytoplasm, poorer in yolk, and in the 

 later stages usually extend a little farther from the animal pole 

 than on the opposite side. In favorable cases this inequahty is 

 revealed by a single meridional section (figs. 20 to 25) ; in other 

 cases it is less readily recognizable through a mental reconstruc- 

 tion of the entire series. 



