100 



GENERAL BIOLOGY 



Lygatu 



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 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 parthenogene- 



Actu.i sis ( ), and in the 



number of V ' ' 



"rf^eMu spmr, E Bf . z M ott.*SS? case ^ tnose 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. 



2. After a varying number of 

 divisions the many new cells thus 

 produced go through a process of 



A. Diagram illustrating the behavior of the g^OWth. They are then called P ri- 

 "accessory," sex-accompanying chromosome in mary OOCVtCS and SpermatOCytCS. 

 fertilization. For the sake of clearness, but rv\\, << 

 four other chromosomes are shown, and these 3. 1 heSC then ripen OT ma- 

 four diagram ma tically ; accessory (x) , solid .. P 1-1 r^-i- . 



black. (After Wilson.) ture," after which fertilization can 



B. A diagram of the gametogenesis and , i i 

 fertilization. take plaCC. 



Premeiot-ic 

 divisions 



