INTRODUCTION 5 



and plant tissues takes place by cell division, and most cell division 

 involves these changes in the nucleus. 



The number of chromosomes, in general, is constant for each species, 

 there being 4 to 30 or more in the body cells of insects. An even number 

 is usual in most animals, but in some forms, especially in the males of 

 certain insects, there may be an odd number. 



B 



,^ 





Fig. 3. — Maturation and fertilization. A, female cell. S, male cell, (o) Oocyte 

 of the first order. {a>) Spermatocyte of the first order, (b) Oocyte of the second order. 

 (6') Spermatocyte of the second order, (c) Egg. (c>) Spermatid, (d) Sperm. (Ip) 

 First, (2p) Second polar body, (e) Egg and sperm. (/) Fertilized egg. 



That the chromosomes maintain their individuahty (genetic con- 

 tinuity) throughout the life cycle has long been held by cytologists. In 

 some plants and animals, the chromosome boundaries do not entirely 

 disappear during the resting stage, from which it is evident that the 

 morphological identity of the chromosome is not lost between mitoses. 

 Chromosomes are the carriers of hereditary qualities from parent to 

 offspring. 



Sex Cells. — During embryonic life the primordial germ or sex cells 

 give rise to numerous cells that migrate into the male or female gonad. 

 The male cells, at this stage known as ''spermatogonia," after a period of 



