REPRODUCTION 



91 



the larger, relatively immobile ovum in order to 

 bring about the mingling of the two substances (Figs. 

 33 and 34). Once the mixing of the two germ cells 

 has taken place, development starts by a process of 

 cell division. The mother cell as the combined male 

 and female germ is now called divides first into two 

 daughter cells (Fig. 35); these again divide, each into 

 two, so that we have four granddaughter cells; then 

 by further division we get eight cells, sixteen, thirty- 

 two, and so on, until a solid ball of cells is formed, in 





sp 



FIG. 34. FERTILIZATION IN Ascaris Megalocephala. (Adapted 

 from Boveri and Van Berieden.) 



A, Spermatozoon (sp) about to penetrate ovum, which has one polar 

 body(Rkl); B, spermatozoon (spk) has entered ovum, which 

 shows ovum-nucleus (Eik) ; also three polar bodies (Rk 1 and 



(From Weismann, "The Evolution Theory.") 



appearance very much like a mulberry and therefore 

 called a morula. This morula undergoes further 

 change. Its central cells liquefy, so that instead of 

 a solid ball there is now a single layer of cells, which 

 encloses a hollow space filled with fluid. This is 

 called a blastula. The blastula then doubles upon 

 itself by growth, the differently situated cells dividing 



