12 10 PHYSIOLOGY 



of chromosomes (Fig. 559). The egg now contains two nuclei or pro-nuclei, 

 exactly similar in appearance, one derived from the male and the other 

 belonging to the egg itself. The two nuclei approach one another and join. 



FIG. 559. Fertilisation and first division of ovum of Ascaris megalocephala. (Slightly 



modified from BOVEBI 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 chromo- 

 somes ; just above it, its archoplasm is shown : the egg- nucleus ( 9 ) also shows two 

 chromosomes. B, both pro-nuclei are now reticular and enlarged ; a double cen- 

 trosome (a) is visible in the archoplasm which lies between them, c, the chromatin 

 in each nucleus is now converted into two filamentous chromosomes ; the centro- 

 somes 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, r, 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. 



In many cases there is an apparent fusion of the substance of the two 

 nuclei. In others the chromatin filaments of male and female simply lie 

 side by side, forming a complete nucleus with the somatic number of chromo- 

 somes. Fertilisation is rapidly followed by cell division. Each of the 



