CHAPTER XXV 



EMBRYOGENY 



The word embryogeny may be defined as the development of an animal 

 from the time when the fertilized egg cell begins to divide until the 

 organism has acquired an organization comparable to that of the adult. 

 Until that time it is an embryo, but afterward it receives different names 

 in the different types of animals. Examples are the larvae of many 

 invertebrates, the tadpoles of amphibia, the chicks of birds, and the 



D E 



Fig. 45. — Diagrams of homolecithal egg cells and total cleavage. A, homolecithal egg 

 cell with nearly uniform distribution of yolk. B, total equal cleavage resulting from the 

 condition shown in A. C, homolecithal egg cell in which the yolk tends to accumulate 

 toward the lower pole. D, total une(iual cleavage of the egg cell shown in C, when the first 

 cleavage plane is horizontal, resulting in the production of two unequal cells. E, cross 

 section of an eight-cell stage, resulting from cleavage in C in a case in which the first two 

 cleavage planes were vertical, giving rise to four equal cells, and the third was horizontal, 

 producing four smaller cells at the upper pole and four larger at the lower. The proto- 

 plasm is stippled, the yolk indicated by the outlines of globules. 



fetuses of mammals. Generally speaking, the higher the place of the 

 animal in the animal kingdom the longer will be the embryogeny. Any 

 particular embryogeny is a part of a corresponding ontogeny, which covers 

 the development of the animal from the beginning until it reaches 

 full maturity. The term embryogeny should not be confused with 

 embryology, which is a broad science covering not only the embryogenies 



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