CHAPTER XII 

 ONTOGENY 



Ontogeny is the development of the individual; phylogeny is the 

 development of the race. Since the history of a race is composed 

 of a number of individual life histories, phylogeny and ontogeny 

 are merely different aspects of the same process, viz., develop- 

 ment. The ovum and sperm are the basic elements from which 

 the individual develops, while individuals in turn constitute the 

 elements of the race. Ontogeny is the subject matter of embry- 

 ology, the study of the development of the individual. In the 

 study of organic evolution interest is focused on the origin and 

 history of the larger unit, the race. 



Germ Cells. — The egg and spermatozoon are alive by virtue of 

 the fact that they are derived from living cells. Reproduction 

 of an individual animal from germ cells is not a creation of life, 

 but a continuation of life in the form of an individual animal 

 body. New individuals are alive because they are produced from 

 living germ cells. The egg is a relatively large cell, whose bulk 

 is due to the presence in the cytoplasm of a greater or lesser 

 amount of yolk stored there during the growth period of oogene- 

 sis; the amount of yolk being correlated with the conditions 

 under which embryonic development takes place. Thus eggs 

 scantily supplied with yolk, such as a human egg (Fig. 145), 

 receive nutriment from maternal tissues during intrauterine 

 development. Other eggs poor in yolk, such as the starfish egg, 

 develop rapidly to a free-living larval stage capable of gaining 

 food from the environment, which is sea water. Large-yolked 

 eggs usually develop outside the body of the mother, the yolk 

 serving as the source of nourishment. Exceptions to this are 

 found in viviparous sharks and snakes, whose large-yolked eggs 

 develop in the uterus. Eggs that are laid are provided with a 

 shell of some sort to protect the developing embryo. The shell 

 is not a part of the egg proper, but is secreted about the egg as it 

 descends the genital tract. Physiologically the egg, before it is 

 stimulated to develop by the sperm or by other means, is a cell 



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