312 ESSENTIALS OF ZOOLOGY 



the number. The reduced condition of the nucleus is called 

 the haploid and the normal number the diploid, and the con- 

 dition is further indicated by stating the number of the haploid 

 as n and the diploid as 2n. The second division maintains the 

 haploid condition of the chromosomes of the nucleus, and in 

 the case of the male for each spermocyte four spermatids result. 

 The three stages are termed spermocyte 1, spermocyte 2, 

 spermatid. In the case of the oocyte the corresponding 

 stages are called oocyte 1, oocyte 2, ovum ; the division is 

 very unequal, a large cell and a small one being produced. 

 The small cells are extruded as polar bodies, and the resultant 

 large cell is the matured egg cell. The spermatids are each con- 

 verted into a spermatozoon. The fusion of the spermatozoon 

 with the ovum, each having the haploid number of chromo- 

 somes, brings into the egg the normal or somatic diploid 

 number, and the 2n condition characterises the divisions of 

 the resulting metazoon and of the contained germ cells until 

 the matured phase, and so on. 



The fundamental change which takes place at the end 

 of the history of the germ cells is evidently of great importance, 

 but it is questionable if we completely understand the meaning. 

 It is clear that it indicates some preparation for the fusion 

 which is imminent, even although only one spermatozoon of 

 countless numbers is successful in gaining entrance. Knowing 

 that an egg may in appearance be very similar to the egg 

 of a totally different metazoon, and yet bears within it the 

 properties which determine not only the kind of animal 

 which will be produced, but even individual peculiarities, it 

 has been natural to correlate the changes with the fact of 

 heredity. It is impossible to go into this and other subjects 

 which are associated with it in such a work as this. It is 

 sufficient to point out to the student the groundwork of the 

 theoretical and experimental work, which has attained to a 

 high degree of importance in recent years. The results are 

 accumulating at such a rate that books are only a partial 

 guide, and periodicals such as the ' Journal of Genetics ' should 

 be referred to. But while the importance of the work is 

 recognised to the full, and without denying the chromosome 



