044 



ZOOLOQT. 



of events : 1. Transportation of the germinal vesicle to the 

 surface of the egg ; 3. An absorption of the membrane of 

 the nucleus or germinative vesicle and a change in the ger- 

 minative spot ; 3. The portion of the nucleus surviving as- 

 sumes a spindle-shape, this portion being largely formed 

 from the nucleolus ; 4. One end of the spindle enters into a 

 protoplasmic prominence at the surface of the egg ; 5. The 

 spindle divides into two halves, one remaining in the egg, 

 the other in the prominence, the latter separating from the 

 egg and forming the polar cell ; 6. A second polar cell forms 

 in the same manner as the first, part of the spindle still re- 

 maining in the egg ; 7. The part of the spindle remaining 

 in the egg, after the formation of the second polar cell, is 

 converted into a nucleus, the temsde pronucleus, and finally, 

 just before fertilization, the female pronucleus takes its po- 

 sition at the centre of the egg. 



Fig. B39. — Development of the sperm-cells of a bliud worm {Epicrium glvtinosum), 

 a, testis-cell ; ft, the same, more numerous ; c, d, e, becoming more nmnerons and 

 finally forming spermatozoa (/). Highly magnified. — After Minot. 



After this, the first step in the development of many-celled 

 animals is the fusion of the protoplasm of the female pronu- 

 cleus with that of the sperm-cell ; for this end the latter is 

 exceedingly minute and provided with a vibratile cilium or 

 " tail," so that it may force its way in toward the centre 

 of the egg. These sperm-cells are developed in the testis 

 of the male. On close examination with very high powers of 

 the microscope, certain cells, called " mother cells," maybe 

 found developed in fine tubules forming the gland ; these are 

 known to possess several nuclei, which are destined to be- 

 come spermatozoa (Pig. 539, a and h) ; these multiply until 

 they become very numerous, elongated, and packed side by 



