XV 



NEMATODA 



449 



e.m 



abnormal relation of the ventral cell family to the descendants of 

 AB had not seriously affected the development. 



Zur Strassen's object in the special study of these T-giants was 

 to elucidate the proximate causes of the orderly succession of all 

 divisions and the orderly movements of the cells on one another, to 

 which the definite form of the body of the embryo owes its origin. 

 Since, even in a T-giant, for instance, C divides evenly into c and y, 

 which lie side by side where C is widely separated from contact with 

 b and f3, it is obvious that the direction of the spindle in this 

 division is in no way influenced, even in the normal embryo, by the 

 neighbourhood of b and /3 to C. 

 To a certain extent the cell C is 

 self-differentiating, i.e. it has the 

 causes of its development within 

 itself, and the question arises 

 whether these causes lie in the 

 cytoplasm or in the nucleus. 



Zur Strassen decides that the 

 causes must lie in the cytoplasm, 

 because the nucleus, both in the 

 resting stage and in the prophases 

 of karyokinesis, is seen to rotate 

 so as to reach its definite position, 

 like a body swept along by a 

 current. The cause he imagines 

 to be an invisible differentiation 

 of the cytoplasm in a definite 

 direction, so that we may conceive 

 its material to be arranged in a 

 series of parallel planes like the 

 cleavage planes of a crystal. 

 These planes, he imagines, com- 

 pel the spindles of the dividing 

 nuclei to be built up either 

 along them or at right angles to 



them, and in turn we might suppose that the planes owed their 

 existence to the effects left on the cytoplasm by the previous spindle. 



If we do so, Zur Strassen shows, by most acute reasoning, that we 

 are driven back step by step till we have to assume that these planes 

 existed in the fertilized egg. The cytoplasm of the fertilized egg 

 would, on this assumption, be of a most complex constitution. Zur 

 Strassen assumes the existence of three sets of cleavage planes at 

 right angles to each other, as well as of several sets of oblique planes, 

 and there is the possibility that this hypothetical constitution would 

 not suffice if he were to carry his analysis of the development to a 

 later stage. 



In order to account for the movements of the cells on one 

 another, which are just as important as the directions of the spindles 



VOL. I 2 G 



FKJ. 352. A "T-giant" of Ascaris megalo- 

 ' cephala seen from the left side. (After 

 Zur Strassen.) 



ect, ectoderm cells descendants of AB; e.m, egg- 

 mem bran c. 



