EMBRYOLOGY OF CRYPTOBRANCHUS 475 



blastic tendency is strongly expressed. This stage is reached 

 about four hours later than the beginning of the preceding stage. 



A description of a few individual eggs will best indicate the 

 characteristics of this cleavage. 



Out of about fifty eggs studied, the one represented in figure 

 97 shows the greatest regularity of cleavage in the upper hemi- 

 sphere. This condition must have been reached by a fairly 

 constant alternation of vertical and latitudinal cleavage furrows. 

 This alternation of cleavage furrows carried out with complete- 

 ness and geometrical precision would give a total of sixty-four 

 cells, consisting of forty-eight micromeres and sixteen macro- 

 meres-; the micromeres would be arranged in three concentric 

 rows, each containing sixteen cells. In the egg under consider- 

 ation, this condition is realized in the outer row of cells, which 

 is quite regular and contains the theoretical number, sixteen. 

 But the total number of micromeres is only thirty-nine, hence 

 a deficiency must exist in the central portion of the blastodisc, 

 or some divisions in this region must have taken place horizon- 

 tally. Sections show that no horizontal divisions ' have taken 

 place in this region; on the other hand divisions parallel to the 

 surface have sometimes occurred in the marginal row of micro- 

 meres. Therefore cell division is taking place more rapidl}^ in 

 the marginal than in the central region of micromeres— a condition 

 which may be the beginning of that accelerated development 

 of the margin, the later expression of which is almost wholly 

 internal. 



A study of other eggs showing a fairly regular alternation of 

 cleavage furrows gives additional evidence for this interpretation 

 (e.g., fig. 100, representing an egg with 39 or 40 micromeres).. 

 While eggs with this degree of regularity in the cleavage pattern 

 are the exception rather than the rule, it is felt that evidence 

 derived from them is especially trustworthy; for in such eggs 

 the equilibrium in the rhythmic alternation of the direction of 

 cell division has been best maintained, and the rather uniform 

 lagging-behind of the divisions of the cells in the central area 

 would seem to be the expression of a normal tendency in the life 

 of the embryo. In eggs which show disturbances in this equi- 



