SUBKINGDOM METAZOA. 47 



which remain, so that the nucleus of the ovum now possesses 

 only half the number of chromosomes which are character- 

 istic for the species. At the time of the formation of the 

 second polar globule the first frequently divides without its 

 nucleus passing into a resting stage, so that as the result of 

 this maturation process four cells have been formed, three of 

 which are small, while the third is relatively very large and 

 will alone undergo further development. When these divi- 

 sions have been completed and the chromosomes have been 

 reduced to one-half their proper number the nucleus of the 

 ovum passes into the resting stage, migrates back towards 

 the centre of the ovum, and is ready for conjugation with the 

 nucleus of a spermatozoon. 



The spermatozoa are always much smaller than the ova, 

 and are, as a rule, capable of active motion, though in certain 

 Crustacea, for instance, they lack this power. The ova and 

 spermatozoa have specialized in opposite directions in this 

 respect. The ova of the Metazoa are specialized as the 

 nutritive cells of conjugation, possessing abundant protoplasm 

 and usually a considerable amount of yolk for the nutrition 

 of the young embryo. They consequently have lost their 

 motility, and in order that conjugation may be made prob- 

 able the spermatozoa lack all unnecessary material which 

 would interfere with their motility, no yolk being stored up 

 and the protoplasm even being reduced to the smallest 

 amount consistent with the development of a locomotor 

 organ. The nuclei, as will be seen later, are essential ele- 

 ments in conjugation, and the spermatozoa are to all intents 

 locomotor nuclei, the ova supplying the protoplasmic nidus 

 necessary for the growth and division of the nucleus formed 

 by conjugation. 



In their typical form spermatozoa are composed of a 

 globular or pyriform head consisting of a nucleus' surrounded 

 by a small amount of protoplasm, and a long filamentous tail 

 continuous with the protoplasm and frequently provided with 

 a delicate fringe-like membrane (Fig. 22, F). By the rapid 

 whipping movements of the tail the organism is propelled 

 through the water, or other fluid in which it may find itself, 

 and so may come into contact with an ovum. 



