EMBRYOLOGY OF CRYPTOBRANCHUS 473 



of cell division at the animal pole. In Cryptobranchus what 

 happens seems to be a shifting of the most active area of cell 

 division to an excentric position in the blastodisc; hence I have 

 preferred to speak of it merely as a process of excentric develop- 

 ment. 



No constant relation exists between the axis of bilateral sym- 

 metry due to excentric development and the original direction 

 of the first cleavage furrow as shown by those portions of it that 

 have not undergone shifting. The two may coincide (fig. 88) ; 

 they may be at right angles to one another; they may be oblique 

 (fig. 87). 



As already indicated, in this stage the cleavage pattern of 

 Necturus bears a strong resemblance to that of Cryptobranchus 

 allegheriensis, but there is this marked difference: the third 

 cleavage furrows of Necturus, when vertical, usually join the 

 first at a greater distance from the vegetal pole, in the region of 

 the equator. In most eggs of Necturus examined in this stage 

 only the first two cleavage furrows extend into the lower hemi- 

 sphere; these usually meet at right angles at the vegetal pole. 

 Thus the cleavage of Necturus in this stage seems to show an 

 even stronger tendency toward the meroblastic condition. But 

 this is merely a consequence of the tendency for the third cleavage 

 furrows to come in obliquely or latitudinally; a comparison of 

 later stages shows that the meroblastic tendency is in reality a 

 trifle less strongly expressed in Necturus (figs. 107 and 108) than 

 in Cryptobranchus. 



In Amia (Dean '96; Whitman and Eycleshymer '97) the fifth 

 cleavage furrows appear in two sets: an outer set cutting the 

 eight macromeres latitudinally; and an inner set cutting the 

 four micromeres in a horizontol plane, hence not visible from 

 the surface. 



Stage 6: (figs. 97 to 102 and 212 to 21 4). This stage is charac- 

 terized by the presence of the sixth cleavage furrow, giving a 

 maximum of sixty-four cells, some of the macromeres being in- 

 completely divided. Considerably more than half the cells are 

 micromeres; these occupy an area whose diameter extends over 

 only about 90° of the circumference of the egg. Hence the mero- 



