ESSENTIAL FEATURES OF THE SEXUAL PROCESS 1261 



FERTILISATION 



The essential features of fertilisation, i. e. the union of the sexual cells, 

 are best studied in some of the lower invertebrates, such as ascaris or echino- 

 derms. In the latter fertilisation takes place in the sea-water, into which 



FIG. 572. Fertilisation and first division of ovum of Ascaris megalocephala. (Slightly 



modified from BOVEEI and WILSON.) 



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

 changed into a reticular nucleus ( 6 ), which however shows distinctly two chromo- 

 somes ; 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 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 centre - 

 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. 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. 



both ova and spermatozoa are extruded. The ovum of the echinoderm 

 consists of a naked mass of protoplasm. Of the countless hordes of sperma- 

 tozoa which may be in the neighbourhood of a given ovum, only one as a 



