FERTILISATION OF THE EGG. 17 



Tlie connection between the formation of polar bodies and 

 the process of fertilisation still remains to be explained. 

 Such cases as those of the gipsy moth and the drone bee 

 indicate that this connection is to be regarded rather as a normal 

 than as a necessary one. Rapid cell division is an exhausting 

 process, and Maupas has shown that in the Ciliate Infusoria the 

 act of fission, which is the most frequent mode of reproduction, 

 although it commences and at first proceeds with great rapidity, 

 after a certain number of generations becomes less rapid, then 

 irregular, and finally ceases altogether. To set it going again, a 

 process of rejuvenescence or constitutional invigoration is ne- 

 cessary ; this is effected by conjugation, during which an inter- 

 change of nuclear matter is effected between the two individuals 

 concerned in the act. 



It seems very possible that the repeated cell division, which 

 takes place in the formation of polar bodies, has a similar ex- 

 hausting effect on the nucleus of the ovum, rendering a process 

 of rejuvenescence desirable, and in most cases absolutely 

 necessary, before any further division can take place ; this 

 rejuvenescence being effected by conjugation, or fusion, of the 

 nuclei of the spermatozoon and of the ovum. 



This view, as Hartog points out, is in accordance with Balfour's 

 theory in so far as it regards the formation of polar bodies as a 

 process the object of which is to prevent parthenogenesis ; but 

 differs from this theory in regarding the polar bodies, not as male 

 elements extruded from an originally hermaphrodite egg, but as 

 cells, the rapid formation of which has reduced the part of the 

 nucleus still remaining in the egg, i.e. the [female pronucleus, 

 to a condition of exhaustion which renders the stimulus of ferti- 

 lisation necessaiy, or at least highly advantageous, if further 

 cell-division is to take place. 



Segmentation of the Egg. 



The actual details of segmentation vary considerably in 

 different cases, the differences depending chiefly on the relative 

 amount of food-yolk present, and on its distribution within 

 the egg. 



The simplest form of segmentation is presented by alecithal 

 eggs, such as those of Amphioxus. It is characterised by the 

 almost geometrical regularity with which the successive divisions 



