64 REPRODUCTION AND LIFE HISTORY. 
divides into the two which play an important réle in the 
cleavage or segmentation of the fertilised ovum. 
(4) When the combined or segmentation nucleus begins 
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 
Fic. 30.—Fertilisation in Ascarzs megalocephala, 
—After Boveri. 
1. Spermatozoon (s.) entering ovum, which contains reduced nucleus 
(1), having given off two polar bodies (4.4. 1 and 2). 
2. Sperm nucleus (the upper), and ovum nucleus (JV), 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. 
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 
nucleus (those of the mother animal), are diffused through- 
out the body of the offspring, and persist in its reproductive 
cells, 
(5) Some eggs, eg. 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 
