FERTILISATION 



75 



(6) Some eggs, e.g. of sea-urchins, can be artificially 

 induced to develop without fertilisation (by being im- 

 mersed for a couple of hours in a mixture of sea-water 

 and solution of magnesium chloride, and by many other 

 means). It seems, therefore, justifiable and useful to 

 distinguish in ordinary fertilisation, (a) the mingling 

 of the hereditary qualities of the two parents, and {b) an 



Fig. 38. — Fertilisation in A^caris megalocephala. 

 — After Boveri. 



1. Spermatozoon (sp.) entering ovum, which contains reduced nucleus 



(N.), having given off two polar bodies {p.b. i and 2). 



2. Sperm nucleus (the upper), and ovum nucleus (A^.). each with two 



chromosomes, and with centrosomes {c.s.). 



3. Centrosomes (c.s.) with " archoplasmic " threads radiating outwards 



in part to the chromosomes of the two approximated nuclei. 



4. Segmentation spindle before first cleavage. 



exciting or liberating stimulus which induces the ovum 

 to divide. 



In one interesting case (the thread-worm Rhabditis) 

 the spermatozoon has the latter function, but not the 

 former ; it enters the ovum and stimulates it to divide, 

 but degenerates without fusing with the egg-nucleus. 

 But in more normal cases, it is found that if the fertilised 

 egg is cut in two before the nuclei have united, the half 

 containing the spermatozoon nucleus may divide and 



