CHAPTER III 

 THE GERM CELLS AND THEIR FORMATION 



THE reproductive elements of the Metazoa are single cells, 

 often greatly specialized in form, and always highly differen- 

 tiated in internal structure and in function. They differ from 

 the reproductive cells of several groups of the Metaphyta, in 

 that they cannot function, i.e., develop, until two single cells, 

 usually derived from two different individuals, shall have met 

 and fused, or conjugated. In many plants single reproductive 

 cells (spores) are formed which develop directly without any 

 such conjugation, and which are therefore to be distinguished 

 from the true germ cells, or gametes, which develop only after 

 conjugation. We shall describe the process of conjugation, or 

 fertilization, in a following chapter, but in order to appreciate 

 the significance of many of the details of germ-cell form and 

 structure, we must remember that they are adapted toward 

 ensuring the conjugation of two unlike cells, egg and sper- 

 matozoon. 



Reproductive cells are set apart from vegetative cells in 

 many of the colonial Protozoa. In some cases they are dis- 

 tinguishable only at certain times, when cells usually vegetative 

 may give up such characteristics and become reproductive; in 

 others the reproductive and vegetative cells remain perma- 

 nently distinct though only slightly differentiated structurally. 

 Finally in a few colonial Protozoa the reproductive cells are 

 considerably modified from the vegetative condition, and in 

 form and composition, as well as in function and behavior, are 

 readily distinguished as germ cells. Many of the details 

 regarding these cells have already been mentioned, others can 

 be more conveniently and more significantly considered later, 

 in connection with the process of fertilization (Chapter V). In 



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