98 



FOUNDATIONS OF BIOLOGY 



water and, entering a cleft in the wall surrounding a large 

 gamete, fuses with it to form a zygote. (Figs. 50, 51.) 



In short, one gamete, designated the EGG, is a large non- 

 motile cell stored with food materials, while the other 

 gamete, or SPERM, is a small active cell largely devoid of 

 food. This is typical of the conditions which are at the foun- 

 dation of gamete differentiation throughout the plant and 

 animal kingdoms eggs and sperm expressing a physiologi- 

 cal division of 

 labor which en- 

 tails structural 

 specialization 

 in opposite di- 

 rections. 



In Oedogo- 

 nium sexuality 

 is apparent both 

 in the behavior 

 and in the 

 structure of the 

 gametes, so 

 that it is pos- 

 sible to identify 

 the sex cells as MALE gametes, or sperm, and FEMALE gametes, 

 or eggs. It will be noted that this is not the origin of sex, 

 for sex arose when spores by their behavior became gametes. 

 In other words, the sex act is the fusion of two cells which 

 reorganize as a single cell; and all modifications of these cells, 

 which enable them to function as gametes, are secondary. 



D. REPRODUCTIVE ORGANS 



Hand in hand with the specialization of spores and gametes 

 there is a progressive modification of the cells or groups of 



FIG. 51. Oedogonium; A, zygote emerging from cell of 

 parent filament. B, division of zygote into four spores. C, 

 mature spores ready to escape and develop into new fila- 

 ments. Note that the zygote does not directly give rise to 

 a filament, but to spores. (From Coulter.) 



