34 HISTOLOGY. 



the ceiiirosome left in the egg divides into two parts (Fig. 13). 

 These centrosomes arrange themselves at the poles of the karyo- 

 kinetic figure which is formed from the chromosomes remaining 

 in the egg. The chromosomes do not split to form the diaster 

 stage. They separate into two groups, giving rise to the two 

 daughter stars, each of which contains one-half the number of 

 chromosomes possessed by the mother star. The whole karyo- 

 kinetic figure becomes situated under the surface of the egg, and 

 a round mass of protoplasm projected from the surface receives 

 one-half of the figure. Thus the extrusion of the second polar 

 body takes place in a way quite similar to that of the first. 

 This completes the process of maturation. 



In consequence of this second division of the egg cell, the 

 egg possesses only half as many chromosomes as other (somatic) 

 cells of the animal body from which the egg proceeds. . Also 

 during the development of the spermatozoon a reduction of 

 chromosomes takes place, so that the ripe sexual cells (the egg 

 as well as the spermatozoon) contain only half the number of 

 chromosomes possessed by somatic cells. Therefore their nuclei 

 have really only half the value of other nuclei. By fertiliza- 

 tion, in which there is a union of the two nuclei containing 

 each one-half of the full number of chromosomes, the normal 

 quantity is restored. 



The process of fertilization i. e., the entrance of the sper- 

 matozoon into the egg begins in many animals not until the 

 extrusion of the second polar body. In the animal under con- 

 sideration the fertilization process is already well advanced at 

 the end of maturation ; for the two processes go on together 

 and begin at the same time. In Physa the whole spermatozoon 

 as a rule enters the egg (Fig. 12). In other animals usually 

 only the head and the middle piece gain entrance. 



Since the function of the tail or flagellum is at an end after 

 the entrance of the spermatozoon into the egg, it undergoes 

 absorption. Around the centrosome of the spermatozoon i. e., 

 the middle piece a new radiation arises in the egg protoplasm 

 (Figs. 13, 14). The radiation and the centrosome of the 

 -pcrmatozoon become more conspicuous at the expense of the 



