106 



General Biology 



this that it is quite possible for the somatoplasm, or outer portion of 

 the body (which is the only portion visible), to cover up many important 

 or, at least, latent and dormant characteristics which an individual 

 may have inherited, but which characteristics may come forth at any 

 moment. In fact one can understand that such characteristics may lie 

 dormant throughout the entire life of a parent and come forth only in 

 the offspring. 



MATURATION AND ELEMENTARY EMBRYOLOGY 



Very low in the scale of life 





Spermatogenesis 



'Gomefocyre 





I 24) 



ReducNon I 

 or 





fl2X2j 



Meiol-.c • 

 division ; 











Posr meiofic 



M 





division 







Cerm cells ( 





D © 0, 



FerKlixed 



or Zygote 



-99 





Ordinary ', 



° r < 

 Premeioric 





( 24 



divisions i 







Fig. 30. 



A. Diagram illustrating the behavior of the 

 "accessory," sex-accompanying chromosome in 

 fertilization. For the sake of clearness, but 

 four other chromosomes are shown, and these 

 four diagrammatically; accessory Or), solid 

 black. (After Wilson.) 



B. A diagram of the gametogenesis and 

 fertilization. 



there is a differentiation into sexes ; 

 the smaller, more active particle is 

 known as the male gamete, while 

 the larger, passive portion is the 

 female gamete. 



In all higher forms fertilization 

 is our starting point in any discus- 

 sion of embryology or development. 



There are apparent exceptions 

 to this rule, such as those insects 

 which give rise to young by virgin 

 birth, a process called parthe- 

 nogenesis ( ), and in the 

 case of those animals in which sev- 

 eral (as high as three) immature 

 generations may be present at the 

 time of birth. This latter condition 

 is known as paedogenesis ( ). 



Before fertilization, various 

 changes take place in the germ cells 

 which are to produce the mature egg 

 and sperm. This process is called 

 maturation ( ), (Fig. 30). 



The early cells are called pri- 

 mordial germ cells. They are in a 

 state of rest in all the higher animals 

 for several years, or until the indi- 

 vidual grows to sexual maturity. 

 When this time has been reached, 

 there are three stages through which 

 the primordial cell passes before 

 producing the mature ovum or 

 sperm. 



1. The primordial germ begins 

 to divide mitotically (Fig. 31). The 



resultant cells are called oogonia and spermatogonia. 



