ANIMAL AND VEGETABLE 



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of the ovum is more complicated than that, for instance, 

 of various kinds of somatic cells, it obeys the same law of 

 alternate repulsion and attraction. 



This may be more readily comprehended by study of 

 the fertilisation and first division of the ovum of the worm 

 Ascaris Megalocephala, owing to the comparative simplicity 

 of the structure and the smaller number of chromosomes. 



To put it, if I can, a little less technically than Schafer, 

 the ovum first discharges or extrudes from its interior two 

 portions of its nucleus, which form globules upon the ovum 

 and are called the polar bodies. These appear to play the 

 same part as the centrosomes and attraction spheres in 

 ordinary mitosis, and, disregarding for the moment the 

 fusion of the male and female pronuclei, the penultimate 

 stages of segmentation of the ovum, as shown by Schafer, 

 differ in no important respect from those of mitotic divi- 

 sion. Those stages are illustrated in the following 

 manner : 



A. Fig. 21. 

 Ascaris Megalocephala. 

 A. Mingling and splitting of 

 the four chromosomes (c) ; the ach- 

 romatic spindle is fully developed, 

 but division of the cytoplasm has 

 not yet commenced. 



B. Fig. 22. 



B. Separation (towards the 

 poles of the spindle) of the halve* 

 of the split chromosomes, and com- 

 mencing division of the cytoplasm. 

 Each of the daughter cells now has 

 four chromosomes ; two of these 

 have been derived from the ovum 

 nucleus, two from the spermatozoon 

 nucleus. 



The extrusion of the polar bodies may be readily under- 

 stood. We know that (1) like electricities repel one 

 another, (2) unlike electricities attract one another, and 



