452 



INVEETEBRATA 



CHAP. 



In the third variety only one of the cells functions as AB, the 

 other three act as P's (Fig. 354, C). The three ventral cell families 

 which result become intertwined with one another in the most varied 

 manner. Some cells eventually become detached from one and added 

 to another, and eventually the irregular mass of cells which results dies. 



Boveri accounts for the occurrence of these three types of embryo 

 by assuming that the diminution of the chromatin which differentiates 



EMST 



EMST 



Fig. 354. — The stage corresponding to the 4-cell stage in the development of the normal 

 egg, whicli occurs in tlie development of the three types of douhly fertilized eggs of 

 Ascaris megalocephala. (After Boveri.) 



A, egg in which three dorsal cell families and one ventral cell family develop. B, egg in which two 

 dorsal cell families and two ventral cell families develop. C, egg in which one dorsal cell family and 

 three ventral cell families develop. 



AB from P is determined by the presence or absence of some peculiar 

 cytoplasmic substance in part of the egg cell, or rather by its presence 

 in greater amount in one place rather than in another. When three 

 AB's and one P are formed, three of the cytocentres are situated in 

 the region which determines diminution and one in the other region ; 

 when three P's and one AB are formed, one of the cytocentres is 

 situated in the region causing diminution and three elsewhere ; and, 

 finally, when two AB's are formed, two cytocentres are situated in this 

 region. 



