XV 



NEMATODA 



453 



In these doubly fertilized eggs a complex spindle is formed con- 

 necting all four centres in the most varied and complex ways, and on 

 the parts of this spindle the various chromosomes, derived from the 

 ovum and from the two male cells, are distributed in the most 

 irregular fashion. Boveri, from a collection of suitable cases, shows 

 that the assumption that the process of diminution is a thing deter- 

 mined by the specific character of some of the chromosomes leads to 

 perfectly untenable positions. The conclusion, therefore, that the 

 cause of diminution lies in the cytoplasm is unavoidable. 



This conclusion is confirmed by the results of some remarkable 

 experiments which Boveri carried out on the normal eggs of Ascaris. 

 When these eggs are subjected to tlie action of severe centrifugal 

 force (about 3000 revolutions per 

 minute), and when the axis of sym- 

 metry of the egg is placed perpen- 

 dicularly to the axis of rotation, then 

 the first cleavage spindle, instead of 

 lying in the axis of the egg, is situated 

 at right angles to it, i.e. tangentially to 

 the direction of rotation, and the egg 

 divides into two precisely equal cells, 

 each of which acts as a P and gives 

 rise to a ventral cell family. The eggs 

 which thus gave rise to two ventral 

 cell families were termed by Boveri 

 "ball-eggs," because a peculiar 

 sphere of granular material, devoid 

 of a nucleus, was found to be ejected 

 from the egg in the direction of a 

 radius of circle of rotation («/, Fig. 

 355). If the axis of the egg lies 

 obliquely to the axis of rotation, then 



the first division, even after violent centrifugal action, gives rise to 

 P and AB cells as usual. 



It follows that, when the peculiar diminution-causing substance 

 is symmetrically distributed with regard to the first spindle, no 

 diminution results, but that when one side of the egg gets a httle 

 more than the other, diminution occurs in one of the two daughter 

 nuclei. 



That the peculiar development of P and AB is in each case due 

 to the peculiarity of the cytoplasm in each of the two cells, and is 

 not due to the mutual reactions of these cells, is proved by another 

 ingenious experiment of Boveri. A large number of developing 

 Ascaris eggs were spread on a slide and covered with a coverslip. 

 On the coverslip a large number of parallel narrow bands of tin- 

 foil were fixed a very short distance apart. When the eggs had 

 reached the 2-cell stage the slide was brightly illuminated by a 

 mercury vapour lamp, the light of which contains a large proportion 



Fig. 3.55. — Stage in the development 

 of a "ball-egg" of Ascaris inegalo- 

 cepJmla corresponding to the 8-cell 

 stage in the development of a 

 normal egg. (After Boveri. ) 

 g', ejected mass of granular material. 



