62 REPRODUCTION AND LIFE HISTORY. 



ovum, the result is usually some abnormality. It is said, 

 however, that the entrance of numerous spermatozoa 

 (polyspermy) is frequent in insects and Elasmobranch 

 fishes. 



(2) The union of spermatozoon and ovum is very intimate ; 

 the nucleus of the spermatozoon and the reduced nucleus of 

 the ovum approach one another, combining to form a single 

 nucleus. 



(3) When this 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 

 orderly as well as intimate, and the subsequent division is 

 so exact, that the qualities so 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. 



As to the interpretation of these facts, Weismann maintains the 

 importance of the quantitative addition which the sperm nucleus makes 

 to the diminished nucleus of the ovum. At the same time, he finds 

 an important source of transmissible variations in the mingling of the 

 two nuclear substances (amphimixis). Others believe that the mingling 

 diminishes the risk of unfavourable idiosyncrasies being transmitted 

 from parents to offspring. Others emphasise the idea that the sperm 

 supplies a vital stimulus to the ovum. 



Segmentation. — The different modes of division exhibited 

 by fertilised egg cells depend in great measure on the 

 quantity and disposition of the passive and nutritive yolk 

 material, which is often called deutoplasm, in contrast to 

 the active and formative protoplasm. The pole of the ovum 

 at which the formative protoplasm lies, and at which the 

 spermatozoon enters, is often called the animal pole ; the 

 other, towards which the heavier yolk tends to sink, is called 

 the vegetative pole. In the floating ova of some fish, how- 

 ever, the yolk is uppermost, and the embryonic area 

 lowest. 



In contrasting the chief modes of segmentation, it 

 should be recognised that they are all connected by 

 gradations. 



