1354 



PHYSIOLOGY 



They diverge, and between them is formed an achromatic spindle, along 

 the equator of which the chromatin filaments of male and female pro- 



FIG. 557. Fertilisation and first division of ovum of Ascaris megalo- 



cephala. (Slightly modified from BOVERI and WILSON.) 

 A, second polar globule just formed ; the head of the spermatozoon is 

 becoming changed into a reticular nucleus ( ^ ), which, however, shows 

 distinctly two chromosomes ; just above it, its archoplasm is shown : the 

 egg-nucleus ( $ ) also shows two chromosomes. B, both pro-nuclei are now 

 reticular and enlarged ; a double centrosome (a) is visible in the archoplasm 

 which lies between them, c, the chromatin in each nucleus is now converted 

 into two filamentous chromosomes ; the centrosomes are separating from 

 one another. D, the chromosomes are more distinct and shortened ; the 

 nuclear membranes have disappeared ; the attraction-spheres are distinct. 

 E, mingling and splitting of the four chromosomes (c) ; the achromatic 

 spindle is fully formed. F, separation (towards the poles of the spindle) of the 

 halves of the split chromosomes, and commencing 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. 



nuclei arrange themselves. It is doubtful, however, how far the centro- 

 some can be regarded as a permanent cell structure. In echinoderm 



