6 4 



REPRODUCTION AND LIFE HISTORY. 



the process of development by dividing, each of the two 

 daughter nuclei which result consists partly of material 

 derived from the sperm nucleus, partly of material derived 

 from the ovum nucleus. In other words, the union is 

 orderly as well as intimate, and the subsequent division is 

 so exact, that the qualities marvellously inherent in the 

 sperm nucleus (those of the male parent), and in the ovum 



FIG. 31. Fertilisation in Ascaris megalocephala. 

 After Boveri. 



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



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



2. Sperm nucleus (the upper), and ovum nucleus (N), 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. 



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

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

 cells. 



(4) 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 

 Lithium chloride, and by other means). It seems, there- 

 fore, justifiable and useful to distinguish in ordinary fertilisa- 



