96 FOUNDATIONS OF BIOLOGY 



Nothing could indicate more clearly the primary relationship 

 of gametes to spores than the origin of sex and sexual repro- 

 duction through the assumption by certain spores of the habit 

 of pairing to form a zygote before germination. 



It should be noted that sexual reproduction is not a differ- 

 ent kind of reproduction, but merely reproduction preceded 

 by the formation of a zygote; a fact very readily lost sight 

 of in the higher forms where accessory phenomena connected 

 with sexuality obscure the essential features, but quite ap- 

 parent in Ulothrix because here the zygote does not form 

 directly a new filament. Instead, after passing a longer or 

 shorter time in a dormant condition protected by a heavy 

 wall, the protoplast (zygote) within divides to form a number 

 of spores, each of which then germinates into a new indi- 

 vidual. Thus in Ulothrix, as in Sphaerella, reproduction is 

 solely by spores, " sexual 'reproduction' not reproducing, but 

 only protecting a spore-forming protoplast." 



C. SEX DIFFERENTIATION 



So far we have seen that sex cells, the gametes, arose with 

 the establishment of the habit of reduced spores uniting in 

 pairs. This is obviously a statement of fact rather than an 

 explanation of sex. Although the two cells which fuse show 

 no morphological characters by which they can be distin- 

 guished from each other, there is certainly a physiological 

 basis of sex which induces them to swim toward each other, 

 to become oriented so that fusion begins at the flagellated 

 ends, and to melt into a single cell, which culminates in a 

 reorganized cell with the complicated structural and physio- 

 logical equipment of the two cells which entered into its 

 make-up. The zygote thus is a cell which combines the 

 characteristics of both the contributing gametes, and to this 



