572 PATTERNS AND PROBLEMS OF DEVELOPMENT 



1 80, B), instead of the right anterior cell, as in rhombus development (Fig. 

 179, E), a reversal of the asymmetry of the dorsal cell group. The adult 

 Ascaris is asymmetrical in that the nucleus of the unicellular excretory or- 

 gan is usually on the left side, but in one individual to about thirty or forty 

 it is on the right, and the proportion of reversed asymmetry in cleavage is 

 about the same; consequently, it is concluded that the asymmetry of 

 cleavage determines that of the adult (Zur Strassen, 1896). 



In spite of the numerous studies on Ascaris development, the relation 

 of egg and cleavage pattern to axiate organismic pattern is still obscure. 

 Apparently egg polarity does not coincide with the longitudinal axis of the 

 animal. Granting this, how is the anteroposterior axis determined? The 

 two dorsal cells, A and B, of the four-cell stage are not distinguishable as 



A B 



Fig. 180. — A, tetrahedral four-cell stage; B, reversed asjonmetry of dorsal cells resulting 

 from tetrahedral form (after Bonfig, 1925). 



anterior and posterior until the change in position results in the rhombus 

 form. Then the cell in contact with P^ is distinguishable as posterior (B of 

 Fig. 179, D) from A, the anterior cell. But the relative change in position 

 of dorsal and ventral cells is not always in the same direction ; it may be at 

 right angles to the usual direction, producing the tetrahedral form, and 

 the two dorsal cells are right and left, instead of anterior and posterior, in 

 relation to the ventral cells and apparently also in relation to the antero- 

 posterior axis of the animal. Is there normally a physiological difference 

 between the two dorsal cells which determines the direction of shift in 

 position in the four-cell stage but which is more or less completely obliter- 

 ated in shghtly inhibiting conditions? If this is the case, how are anterior 

 and posterior end determined in the tetrahedral forms? Or are the two 

 dorsal cells alike, and does the shift occur indifferently in either direction 

 to form the rhombus and at right angles to these directions to form the 

 tetrahedron? If this is the case, the ventral cells apparently determine the 

 anteroposterior axis. 



