THE GENERAL, SCHEME 



The egg or ovum. We have already mentioned the simple 

 fresh-water polyp Hydra as an example of animals that 

 reproduce by budding. In this same animal, however, there is 

 another kind of reproduction that occurs from time to time, 

 in which the living part that is detached from the parent 

 to form the new generation is not a bud, made of many cells 

 and resembling the parent, but a single cell. As shown in the 

 diagram (Fig. 5, left) and the photograph (Plate II, A) 

 from time to time one of the cells in or near the surface of 

 the animal enlarges very much and stores up materials with 

 which it can be nourished for a while after it is cast off from 

 the parent. This is the egg cell or ovum. The few cells that 

 surround it where it grows on the side of the animal could be 

 called an ovary (as we call the organ of similar function in 

 higher animals) if it were worth while to dignify so simple 

 and transitory a structure as the egg hillock of Hydra by 

 considering it an organ. An egg, then, is a simple cell that is 

 set aside by the parent and destined to divide into many cells 

 and thus become an adult animal after the fashion of its kind. 

 Seen in this light, reproduction by means of an egg is merely 

 another case of reproduction by fission, in which the two 

 living products of division are very unequal, the egg on one 

 hand, the maternal animal on the other. If we compare a 

 Hydra and its egg with an animal of a single cell, say a 

 Cothurnia, that is going to divide, we see that the animalcule 

 though an adult has also the function of an egg, for it can 

 give rise, by division, to another animal body. In short, in 

 one-celled animals the same cell must necessarily carry on all 

 the functions of life, including reproduction ; in many-celled 

 animals the function of reproduction can be delegated to 

 special cells. 



The Scholastics debated which came first, the hen or the 

 egg. Modern biology has an answer: they were contempo- 

 raneous ; among protozoans the hen is the egg. Neither came 

 first ; they merely became distinguishable whenever it was (the 



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