CHAPTER X 



EMBRYOLOGY 



Although knowledge of the habits by which animals insure the occur- 

 rence of reproductive processes and the maintenance of their young 

 during critical periods is much of it comparatively new, the principal 

 steps by which the fertilized egg is transformed into an animal in the form 

 of the adult have long been known. The field of embryology has been 

 diligently explored, and knowledge of embryonic development is corre- 

 spondingly definite. 



An embryo, strictly speaking, is an undeveloped animal while still in 

 the egg membrane, in the case of oviparous or ovoviviparous species, or 

 while attached to the maternal uterus, in viviparous animals. One might 

 expect, theixcfore, that embryology would deal only with that part of 

 developmentHaking place after the fertilization of the egg and prior to 

 birth or hatching. In practice, however, embryology is more inclusive. 

 It has been pointed out in the preceding chapter that animals may be 

 hatched or born in very immature stages. In some of them certain organs 

 characteristic of the adult are not yet present at the time of hatching, or 

 the young animal may have organs which the adult does not possess. 

 Such immature animals are spoken of as larvae. The transformation of a 

 larva into the adult form, a phenomenon known as metamorphosis, is 

 usually included in embryology. 



The germ cells, moreover, undergo a characteristic development 

 before they are capable of union in fertilization, and hence before any 

 embryo is produced. This process of development in the germ cells 

 is independent of the special breeding habits of the animal, and is reck- 

 oned as a part of embryology. In the term embryology, therefore, is 

 comprehended the entire development of the germ cells, and of the 

 embryo which they produce, up to the time of birth or hatching, or 

 until such later time as the organs of the adult are laid down and any 

 temporary larval organs arc lost. 



Early Origin of Germ Cells. — The differentiation of cells into two 

 general classes, somMic cells which are sterile, and germ cells capable of 

 reproduction, has been described in Chapter V. It was shown that 

 a difference between sterile and reproductive cells is found in aggregations 

 which are otherwise very simple in structure, such as Pleodorina, whereas 

 differentiations other than that between somatic and germ cells appear 



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