EMBRYOLOGY OF CRYPTOBRANCHUS 477 



beneath sometimes appear between the cells. Evidently the 

 roof consists of a single layer of flattened cells; this inference is 

 confirmed by the study of sections. 



On account of the slow cleavage and relative stability of the 

 macromeres, there is little change in the cleavage pattern of the 

 lower hemisphere. An advance is shown in that the vertical 

 fifth cleavage furrows have invaded the lower hemisphere (fig. 

 102), Those fourth cleavage furrows that proceed vertically are 

 seldom complete in this stage, but sometimes are found joining 

 an earlier vertical furrow at a considerable distance from the 

 vegetal pole. 



In this stage the most regular type of cleavage pattern of Cryp- 

 tobranchus bears a striking resemblance to the corresponding 

 stage of Amia (Dean '96; Whitman and Eycleshymer '97). 



Since Stage 6 of the egg of Cryptobranchus best serves to illus- 

 trate the fundamental characteristics of the cleavage pattern, 

 particularly with regard to the relative size of the micromeres 

 and macromeres, at this point a comparison may well be made 

 with the dipnoi and crossopterygii . The general anuran or urodele 

 character of the cleavage of the dipnoan egg is apparent in all 

 existing genera: Ceratodus (Semon '00 and '01); Protopterus 

 (Budgett '01; Kerr '09); and Lepidosiren (Kerr '00, '01 and '09). 

 With respect to inequality in the cleavage, Lepidosiren in partic- 

 ular closely approaches the condition in Cryptobranchus and 

 Necturus. The cleavage of Polypterus (Kerr '07) bears a general 

 resemblance to that of Amblystoma and the frog. 



Stage 7: {figs. 103 to 105; 215 and 216). This stage is charac- 

 terized by a doubling of the number of cells found in the preceding 

 stage, and by a slight extension of the region occupied by the 

 micromeres. The stage is reached about four hours later than 

 Stage 6. 



Figures 103 and 104 show a fairly representative egg in this 

 stage. The cells in the region of the animal pole are markedly 

 larger than the other micromeres. This condition may be due 

 to one or both of two factors : (a) the flattening of the cells com- 

 posing the roof of the segmentation cavity; (b) a slower rate of 

 division in these cells, as noted in Stage 6. There is marked 



JOURNAL OF MORPHOLOGY, VOL. 23, NO. 3 



