EMBRYOLOGY OF CRYPTOBRAXCHUS ALLEGHEXIENSIS. 157 



are rather difficult, on account of the faintness of the cleavage 

 lines in this region. Material was preserved at intervals and 

 camera drawings afterwards made of the principal stages ; but for 

 the earliest stages the freehand sketches made from living material 

 are sufficiently accurate, and have been used in preference to 

 camera drawings from preserved material. 



Some of the eggs found on September 7 were left to develop 

 in situ as a check upon those studied under artificial conditions ; 

 but so far as cleavage is concerned no difference could be detected 

 between these eggs and those kept in a dish of shallow water. 



The abundance of the material at hand made it possible to 

 secure a series of drawings of the segmentation stages which 

 should be fairly representative. The drawings include some of 

 the most regular and geometrically perfect figures that could be 

 found, and probably these may be regarded as typical ; but in 

 Cryptobranchus as in other amphibians the amount of variation is 

 great. Since departures from the type are not necessarily the 

 result of pathological conditions, they should not be dismissed as 

 abnormalities ; for they may be the expression of opposing factors 

 in the development of the egg. Such factors are the proportion 

 of yolk to protoplasm, the qualitative composition of the yolk 

 determining the extent to which it responds to the sorting influence 

 of gravity, and hereditary factors in the protoplasm ; variations in 

 these factors affect the manner of cleavage, and according as one 

 tendency or another prevails we may find the typical form or 

 variations from it in definite directions. Hence the figures include 

 representations of eggs in the same stage showing differences in 

 the method of cleavage. There are of course individual differ- 

 ences in the rate as well as in the manner of cleavage. 



A. The First Cleavage Furrow. In eggs laid September 6 at 

 about 6:30 P. M., there was so much diversity in the time of 

 appearance of the first cleavage furrow that it is difficult to assign 

 limits. Probably more cases of first cleavage were noticed at 

 about 10 A. M. of the next day I 5 hours after the eggs were 

 laid than at any other time. In eggs laid during the night of 

 September 78 the first cleavage furrows appeared in several eggs 

 at about 5 P. M. of the next day, and cases were numerous at 

 6:30 P. M. The eggs found on September 7 at 10 A. M. were 



