CHAPTER LXIII 



DEVELOPMENT OF THE ORGANISM 



EMBRYOLOGY 



In Chap. X the subject of reproduction in animals was first presented 

 and it was there stated that while in the most primitive of Protozoa repro- 

 duction occurs by simple division of the single cell which makes up the 

 organism, in the higher types of Protozoa and in Metazoa certain cells 

 known as gametes function as reproductive cells. The latter was spoken 

 of as sexual reproduction. In Protozoa, however, a zygote so formed is a 

 one-celled animal and so the process of reproduction does not involve the 

 formation of an embryo. In the many-celled animals the zygote gives 

 rise to the metazoan body only after repeated cell division, and the 

 structures of the adult are gradually formed. During this time differen- 

 tiation and integration occur and the organism passes through a series 

 of developmental stages during which it is known as an embryo. The 

 study of such stages provides the subject matter of embryology. 



482. Germ Cells. — Strictly speaking, embryogemj begins with the 

 zygote, but since many of the phenomena concerned with early embryonic 

 development are directly traceable to those which attend the production 

 and maturation of the germ cells, the development of these cells is usually 

 taken as the starting point in this field. In Chap. XXIII has been con- 

 sidered the development of the germ cells, or gametogenesis, which 

 includes both spermatogenesis and oogenesis. 



483. Origin of Germ Cells. — The difference between the germ cells and 

 somatic cells in animals may usually be detected early in the life of the 

 individual. In the case of the arrow worm, Sagitta, the egg cell possesses 

 a so-called x-body. In the 4-cell stage this body still remains in one of 

 the cells and this continues until the 64-cell stage. The cell which then 

 contains this a:-body may be identified as the primordial germ cell; from 

 the other cells are developed the somatic cells of the body. In Ascaris 

 the primordial germ cell may be recognized as one of the cells in the 

 four-cell stage. In certain insects the primordial germ cells have been 

 recognized very early by their large size and peculiar structure. In 

 vertebrates the primordial germ cells are not recognizable until consider- 

 ably later, but still early in embryonic life cells lodged among those lining 

 the digestive tract may be seen to migrate to a place in the wall of the 

 coelom, where they accumulate and form the beginning of a reproductive 

 organ. These cells are recognized as the primordial germ cells. 



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