CHAPTER II. FECUNDATION; MOTILE ISO- 

 GAMETES. 



ULOTHRIX AND HYDRODICTYON. 



There seems to be no question that the simplest and most primitive 

 form of sexuality consists in the union of motile isogametes as found 

 among many of the most primitive algse. The chief difference be- 

 tween the gametes of such forms as Pandorina and Ulothrix, for 

 example, and their asexual swarm-spores, from which the gametes 

 were undoubtedly derived phylogenetically, seems to be merely phys- 

 iological. Generally speaking, the gamete is incapable of developing 

 into a normal adult individual. It must unite first with another gamete 

 of the same species in order to restore the power of growth and divis- 

 ion necessary to the development into an individual common to the 

 species, and apart from theoretical considerations (I refer to the num- 

 ber of chromosomes which, of course, has not been determined for 

 these lower forms) this is the most fundamental distinction made. 

 Many other well-known forms among the green algae might have been 

 taken as representatives, instead of the two selected, but these have 

 been chosen because the development of the reproductive cells from 

 the mother-cell has been more carefully worked out here, and because 

 the processes in this development are coming to be regarded as more 

 important from a genetic standpoint. 



In connection with Ulothrix I have selected Hydrodictyon in order 

 to present the cytological processes preparatory to the formation of 

 gametes in uninucleate as well as in multinucleated cells. 



The cytological development, leading to the formation of gametes 

 and also asexual swarm-spores among the simpler representatives of 

 the green alga?, has been investigated by a number of earlier observers, 

 among whom were Alexander Brown, Cohn, Pringsheim, Dodel, 

 Strasburger, Klebs, and lately by Timberlake. 



The well-known and widely distributed Ulothrix consists of a simple 

 unbranched filament differentiated into base and apex (Fig. 17, A). 

 The cells, except the basal one, which is modified as an organ of 

 attachment, are quite alike. Each contains a single nucleus and a 

 band-shaped chloroplast in the form of an almost complete hollow 

 cylinder. Almost any vegetative cell of the filament save the basal 

 one may, without undergoing any external modification, function as a 

 gametangium. 



