THE MASTIGOPHORA 265 



The syngamy of Copromonas is thus seen to be a case of perfect 

 isogamy, and is probably to be regarded as representing a very 

 primitive type, whence the more complex sexual processes of other 

 Flagellata have been evolved (1) by greater specialization and 

 differentiation of the gametes in their relation to other phases of 

 the life-cycle (gamete-formation) and to one another (sexual differ- 

 entiation) ; (2) by correlation of the sexual phases with definite 

 crises, to which they become restricted, in the general life-cycle. 



In the Rhizomastigina sexual processes occur of a type resembling those 

 found in the Sarcodina to such an extent as to indicate that the affinities of 

 this group is rather closer to some of the primitive Rhizopods than to typical 

 Flagellata. The life-cycle (Fig. 112) has been worked out in full detail in 

 Mastigdla vitrea by Goldschmidt (41). Vegetative reproduction in the free 

 state is by binary fission of the ordinary type, and occurs when food is abun- 

 dant ; a falling- off in the supply of nutriment leads to gamete -formation and 

 syngamy. In the earliest stages of the sexual generation a differentiation 

 of the individuals into macrogametocytes and microgametocytes is to be 

 observed, though externally they are similar to ordinary individuals and 

 continue their vegetative life during the early stages of gamete -formation. 

 In the macrogametocyte, first a quantity of nucleolar substance, and then of 

 chromatin, is set free from the nucleus ; these two substances unite to form 

 a chromidial mass from which a nunlber of secondary nuclei are formed. 

 The secondary nuclei become scattered through the cytoplasm, and each 

 becomes surrounded by a protoplasmic body. The small cell thus formed 

 is a macrogamete, which goes through reducing divisions. The still active 

 macrogametocyte, which has its cytoplasm crammed with the small gametes, 

 now becomes encysted. Within the cyst the gametes acquire flagella and 

 become motile. At this stage the original nucleus of the gametocyte breaks 

 up and disappears rather suddenly. Finally the cyst-wall is ruptured and the 

 flagellated gametes escape. 



The formation of the microgametes takes place in a manner essentially 

 similar to that already described for the macrogametes, but with a few 

 differences in detail. The microgametocytes become encysted at the very 

 beginning of the process ; then formation of chromidia begins, and as soon 

 as it is completed the primary nucleus degenerates ; the microgametes have 

 no flagella, and are shot out of the cyst when it bursts. 



The free macrogametes measure on the average about 3'6 /z diameter, 

 and have a flagellum 15 to 18 p. in length ; the microgametes are 2*8 p. in 

 diameter, and have no flagellum. A macrogamete seeks out a microgamete 

 and fuses with it, cytoplasm and nucleus. The zygote retains the flagellum 

 of the macrogamete, and becomes a small, monad-like individual which 

 multiplies by fission as such. After several generations the monads cease 

 to multiply, and each grows up into an adult Mastigdla. A development 

 similar in the main is described by Goldschmidt for Mastigina, but some of 

 the phases escaped his observation. 



Comparing the sexual cycle of Mastigdla (Fig. 112) with that of 

 Copromonas (Fig. Ill), the chief difference is seen to be that in 

 the former an ordinary individual does not become a gamete directly 

 but a gametocyte, which by a process of multiple fission gives rise 

 to a generation of minute swarm-spores, the gametes. In the two 

 sexes a slight differentiation of the gametes is seen. Further, in the 

 life-cycle of Mastigdla considered as a whole, there are two forms 

 of individuals, each capable of multiplying vegetatively for many 



