64 REPRODUCTION AND LIFE HISTORY. 



nucleus (those of the mother animal), are diffused through- 

 out the body of the offspring, and persist in its reproductive 

 cells. 



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

 induced to develop without fertilisation (by being immersed 

 for a couple of hours in a mixture of sea water and solution 

 of magnesium chloride, and by other means). It seems, 



FIG. 30. Fertilisation In Ascarts wegaJocephala. 

 After Boveri. 



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



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



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



chromatin elements or idants, 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. 



therefore, justifiable and useful to distinguish in ordinary 

 fertilisation, (a) the mingling of the hereditary qualities of 

 the two parents, and (b) an exciting or liberating stimulus 

 which induces the ovum to divide. 



It should be noted that the chromosomes of the spermatozoon do not 

 fuse with the chromosomes of the ovum when fertilisation occurs. 

 They are associated together and divide together in all the cell- 

 divisions, whether of body-making or of the germ-cell lineage. In 

 some of the divisions of the germ-cell lineage there seems to be an 



