34 J. BRONTE GATENBY 



shown by Van Beneden, Boveri, Wilson, Morgan, Montgomery, 

 McClung, Doncaster, and many othors, to go through certain 

 definite changes, which have been found to correspond with 

 many of the pecuhar phenomena of sex and heredity in breeding 

 experiments. The main facts ascertained with regard to the 

 chromosomes are briefly as follows : 



1. They are constant in number in any one species. 



2. In ordinary cell-division each chromosome is halved so 



that each moiety is a complete replica of its fellow. 



3. In the formation of the germ-cells there is a process 



whereby the ripe gamete comes to have the halved or 

 haploid nundjer of the chromosomes. 



4. The male and female pronuclei in fertilization are 



practically equivalent, and possess the same number of 

 chromosomes (overlooking the x and y chromosomes). 



5. In the formation of the ripe spermatozoon no visible part 



of the chromatinic substance is rejected. 



In the cytoplasm of the animal cell it has been shown that 

 two important categories of formed protoplasmic elements 

 exist : namely, mitochondria and Golgi elements. The purpose 

 of this section is to compare and contrast the behaviour of 

 these protoplasmic bodies with the chromosomes of the nucleus. 

 Under the first heading — ' That the chromosomes are constant 

 in number in any one species ' — we may compare and contrast 

 the Golgi body and mitochondria. While it is not generally 

 possible to gain absolutely explicit evidence by examining 

 the mitochondria in most animals, it is nevertheless true 

 that in some forms the mitochondria are so few and so large that 

 definite counts may be made. As examples I give the follow- 

 ing : (a) In Paludina the typic spermatid may contain from 

 four to seven spheres. Four is the commonest number. These 

 spheres are suliequal in size in those spermatids which contain 

 four spheres and in those which contain seven, (h) Wilson (30) 

 has shown the same variation to apply in Centrurus. and Retzius 

 (25) also in a variety of Molluscs, (c) It was shown (9) that 

 in Helix aspersa the mitochondria in one spermatocyte 



