PROCESSES RELATED TO SEXUAL REPRODUCTION 223 



of each was long in dispute, the " spermatists " of the eighteenth 

 century maintaining that the embryo arose from the sperm, the 

 " ovists " that it came from the ovum. The answer to the riddle 

 was found only as the result of two discoveries: the recognition 

 that the ovum and spermatozoon are cells, following the estabhsh- 

 ment of the Cell Theory in 1838-39; and the discovery by Oscar 

 Hertwig, in 1875, that fertihzation consists in the entrance of a 

 single spermatozoon into the egg and the union of egg-nucleus with 

 sperm nucleus. An animal was thus shown to begin its existence 

 as an individual at the time of fertilization, and development was 

 recognized as a problem of cell division and differentiation. The 

 gametes were seen to be the basis of the hereditary transmission 

 of characters from one generation to another and the most 

 remarkable of all cells because of their potentialities. 



Confusion will be avoided if we recall the terms that must be 

 used in this connection. The word ovum is applied to the female 

 gamete of an animal in the final stages of its development and 

 before fertilization, in contrast to the male gamete, which is called 

 the spermatozoon. The word " egg " is used more loosely, as when 

 one speaks of a hen's egg, which is a complex structure including 

 the shell and " white," which are secreted by the oviduct, and the 

 ovum or " yolk," which is a product of the ovary; or, biologically 

 speaking, the word egg may mean the ovum. The word gamete is a 

 general term for both male and female sex cells. The germ cells 

 include not only the full}' fomied ova and spennatozoa, or gametes, 

 but also the antecedent cells from which these gametes are differ- 

 entiated. Since the germ cells may be recognized at early stages 

 of development as the primordial germ cells that are seen in many 

 animals (Fig. 112), the distinction that can be made on theoretical 

 grounds between germplasm, or germ cells, and somatoplasm, or 

 body cells, is actually recognizable in particular cases. What 

 is known as the germ-cell cycle includes the entire history of the 

 germ cells from the time they can be recognized in the embryo 

 until they have become differentiated as ova and spermatozoa. 



Origin of the Germ Cells. — The relationship between the germ- 

 plasm and the somatoplasm is frequently illustrated by such a 

 diagram as that shown in Fig. 111. According to the theory that 

 is commonly associated with this representation, the germplasm is 

 of primary importance, since it gives rise to both body cells and 

 germ cells in each generation, while the somatoplasm is destined 



