166 



THE LOWEST ORGANISMS AND THE CELL 



The egg is a female sexual element which normally cannot 

 develop into a new organism until a male sexual cell, called 

 the sperm, has united with it. This union is called fertilization 

 and the fertilized egg is a sexually formed cell because it results 



from the fusion of two sexual cells, 

 the egg and sperm. The fertilized 

 egg is termed an oospore (meaning 

 an egg spore), when there is a rest- 

 ing period before its further develop- 

 ment, or germination. A sexual cell, 

 such as the egg or sperm, is called 

 a gamete. The protoplasmic union 

 of egg and sperm is very complete, 

 for the two nuclei come together in 

 the center of the egg and fuse to 

 form a large nucleus which has, of 

 course, about twice as much of that 

 important nuclear substance, chro- 

 matin, as the single nucleus of either 

 egg or sperm. 



Frequently there are present in 

 FIG. 171. The union of the J 



gametes in Spirogyra plants other forms of reproductive 



A, two filaments of Spiroyyra ceUs ^^ S P reS > which are not 

 quinina, side by side, show- formed sexually but are simply spe- 



ing stages in the union of the -, n r -r j i 



cells (gametes) to form the Cial cells whicn Can develop at Once 



zygospores ; B, another species into new plants. 



(S. longatd), in which the cell mi . . 



unions occur between adjacent The union of gametes to give 



gametes in the same filament. sexually formed cells is especially 



After Schenck ,, .,. , . , . . . , ,, 



well illustrated in the fruiting 1 of 



the pond scum (Spirogyra). In most species the cells of fila- 

 ments lying side by side put forth short processes which fuse in 

 pairs, thus presenting a characteristic ladder-like arrangement 

 (Fig. 171, A). The contents of one cell then pass over and 



1 The terms fruit and fructification will be used in Part II in an untech- 

 nical sense to designate various forms of reproductive organs and processes. 





