THE CELL 



256 



Hence this chapter may be divided into two sections : the Mor- 

 phology and the Physiology of the process of fertilisation. 



I. The Morphology of the Process of Fertilisation. 

 Up till now the process of fertilisation has been thoroughly worked 

 out to the most minute details in three objects : in the animal egg, 

 in the embryo-sac of Phanerogams, and in Infusoria. Although 

 these three objects belong to different kingdoms of the organic 

 world, they show a marked similarity in all the processes 

 peculiar to fertilisation. It is, therefore, most suitable to com- 

 mence this section by investigating these three objects. We will 

 then occupy ourselves with the more general points of view pro- 

 vided by a study of comparative morphology, discussing : 



1. The different forms of sexual cells, the relative importance 



t>6 /c<H 



FIG. 138. A, B, C small sections from the eggs of Asterias glacial-is (after Fol). The 

 spermatozoa have already penetrated into the gelatinous sheath covering the ova. In 

 A a protuberance is commencing to raise itself to meet the most advanced spermato- 

 zoon. In B the protuberance and spermatozoon have met. In G the spermatozoon has 

 entered the ovum. By this time a yolk membrane with a funnel-shaped opening has 

 developed. 



of the cell-substances, which are concerned in the generative act, 

 and the idea of " male and female sexual cells." 



2. The original and fundamental forms of sexual generation, 

 and the derivation of sexual differences in the animal and vege- 

 table kingdom. 



1. Fertilisation of the Animal Egg. Echinoderm ova 

 (Hertwig VI. 30 ; Fol. VI. 19, VII. 14) are classical subjects for the 

 study of the processes of fertilisation, as also are the eggs of 

 Ascaris megalocephala (van Beneden VI. 4 a, 4b; Boveri VI. 6, etc.). 

 They complement each other, for some phases of the process 

 are more easily to be demonstrated in the one, whilst others are 

 more plainly to be seen in the other. 



