CHAPTER 16 

 EMBRYONIC DEVELOPMENT 



The minimum accomplishment of the reproductive processes is the 

 formation of germ cells. With the aid of breeding behavior these germ 

 cells are brought together in a favorable environment, where they are 

 gradually converted into new organisms. Into this period of transfor- 

 mation of the fertilized egg into an active independent being is crowded a 

 multitude of changes — analyses, reconstructions, rearrangements, growth, 

 and differentiations — which constitute embryonic development. Embry- 

 ology may properly treat of many of the things already described as 

 breeding habit or reproduction; but there is left for examination in this 

 chapter the whole series of structural changes and the chains of physi- 

 ological events which lead to the formation of the new individual. The 

 story may begin with the reorganization of the reproductive or germ cells. 



Maturation of the Germ Cells. — The germ cells in a very young 

 animal may remain for a long time in a relatively undifferentiated con- 

 dition. Often it cannot even be stated whether they will become eggs 

 or spermatozoa, yet in most animals, despite theu' lack of recognition 

 marks, they are irrevocably destined to become the one or the other. 

 During this time they divide frequently by ordinary mitosis, thereby 

 multiplying in number. In this apparently unspecialized condition the 

 reproductive cells are called, in a male animal, spermatogonia (singular, 

 spermatogonium), in a female, oogonia. 



As the time of reproduction approaches, the spermatogonia and 

 oogonia undergo a series of remarkable changes called spermatogenesis and 

 oogenesis, respectively. These changes consist typically of two rapidly 

 succeeding cell divisions, in one of which the number of chromosomes is 

 reduced to half. There are many variations in the process in different 

 species, but the fundamental features are the same for nearly all the 

 higher animals. 



Spermatogenesis. — As soon as the spermatogonia reach the end of 

 their multiplication period, that is, as soon as they have divided by 

 ordinary mitosis for the last time, the cells are known as primary sperma- 

 tocytes. The history of these cells in their further development is illus- 

 trated in Fig. 164, to which constant reference should be made throughout 

 the following account. 



During all of their history up to this time, the germ cells contain the 

 same number of chromosomes as any other cells of the body. That 



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