58 REPRODUCTION- AND LIFE-HISTORY OF ANIMALS. 



As to the interpretation of these facts, Weismann main- 

 tains the importance of the quantitative addition which the 

 sperm-nucleus makes to the diminished nucleus of the ovum. 

 At the same time, he finds the immediate source of all the 

 variations in animals in that mingling of two nuclear plasmas 

 which occurs as we have noted, in fertilisation. 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 yolk tends to sink, is called 

 the vegetative. 



We shall now contrast the chief modes of segmentation, 

 but it should be recognised that they are all connected by 

 gradations. 



A. Complete Division — Holoblastic Segmentation. 



I. Eggs with little and diffuse yolk-material divide com- 



pletely into approximately equal cells, 

 [or. Ova which are alecithal (i.e., without yolk) under- 

 go approximately equal holoblastic segmentation]. 

 This is illustrated in most Sponges, most Coelen- 

 terates, some " worms," most Echinoderms, 

 some Molluscs, in all Tunicates and in Am- 

 fhioxus, and also in most Mammals. 



II. Eggs with a little yolk-material accumulated towards 



one pole, divide completely, but into unequal 

 cells, 

 [or. Ova without very abundant deutoplasm, but with 

 what they have lying towards one pole (telolecithal), 

 undergo unequal holoblastic segmentation]. 



