EMBRYOLOGY OF CKYPTOBRANCHUS 555 



These stages seem to possess few characters intermediate between 

 the extremes indicated, consequently they tell us little about 

 the relationships of the larger groups; but the coenogenetic char- 

 acters, which are plastic and vary widely within a limited range, 

 may be of value for determining relationships within these groups. 

 The most striking of these coenogenetic characters are correlated 

 with the presence of yolk (Conklin '07) . 



In the highly telolecithal and heavily yolk-laden egg of Crypto- 

 branchus, we may interpret holoblastic cleavage as a persistent 

 primitive character. For were the ancestral form one with mero- 

 blastic cleavage, we should hardly expect the holoblastic method 

 to arise under such unfavorable conditions. 



Comparisons between the early cleavage furrows of different 

 urodeles seem justified on the ground that we are comparing cells 

 of the same generation; but in view of the highly indeterminate 

 character of the cleavage (Jordan and Eycleshymer '94) such 

 comparisons do not take us very far. Mechanical factors 

 doubtless play a part (McMurrich '94), but these mechanical 

 factors are conditioned by the organization of the egg, which is 

 hereditary. 



There is close correlation between the method of third cleavage 

 and the yolk content. A vertical third cleavage is characteristic 

 of heavily yolk-laden and highly telolecithal eggs; a latitudinal 

 third cleavage is found rather in eggs with yolk both smaller 

 in amount and with a lesser degree of segregation from the cyto- 

 plasm. Morgan ('93) has shown that in teleost eggs from which 

 yolk has been experimentally removed, the third cleavage often 

 comes in latitudinally, yet the eggs produce perfect embryos. 

 Marked variation in the direction of the third cleavage furrows 

 occurs in eggs in which the conditions are intermediate in char- 

 acter; the egg is oscillating between two possible modes of 

 cleavage. A vertical third cleavage is characteristic of the egg 

 of Cryptobranchus; it is found less uniformly in the eggs of 

 Desmognathus; in Necturus and Diemyctylus the third cleav- 

 age is irregular; in Amblystoma it is latitudinal. So far as this 

 character is concerned Cryptobranchus lies nearest to Desmog- 

 nathus and is most remote from Amblystoma. 



