226 



THE PROTOZOA 



and by most authors are called sporoblasts. The so-called sporoblasts 

 become motile and move about with considerable freedom, a hitherto 

 unrecognized phenomenon. But more remarkable still, the sporo- 

 blasts finally unite two by two, and after complete fusion of nuclei 

 and cell-plasm, each double cell or copula divides into eight parts, 

 the sporozoites (Fig. 124). These observations, which are the most 

 conclusive that have yet appeared, place the Gregarinida in line with 

 the Reticulariida and Radiolaria, and Siedlecki with Mesnil ('oo) sees 

 in this isogamous union a feature which distinguishes the Gregarinida 

 from the Coccidiida. 



The obligatory fusion of microgonidia is widely distributed in the 

 colonial forms of Mastigophora, especially in the Phytoflagellida. It 

 is to be regretted that we do not know the full life-history of the 



Fig. 124. Conjugation of Monocystis acidia Lank. [SIEDLECKI.] 



The two gregarines unite (A). The nuclei divide repeatedly, and many gametes are 

 formed (B). These unite two by two, forming spores. Each spore divides to form eight sporo- 

 zoites (c). 



simpler and more indefinite colonies such as Dinobryon, Anthophysa, 

 or Symira, where the aggregate arises through continued binary 

 division, for in the higher types, the fertilized egg, as in Metazoa, 

 passes by a regular cleavage into the adult form, the cells becoming 

 separated only in the later stages. Nevertheless, in these more 

 differentiated types some stages are unquestionably more primitive 

 than others. In Gonium pectorale, a colony consisting of sixteen in- 

 dividuals, the colonies reproduce asexually by simultaneous division 

 of all of the cells, four successive longitudinal divisions in each cell 

 resulting in sixteen groups of sixteen cells each, and these groups 

 form independent colonies (O. F. Muller, Cohn, Stein Fig. I25). 1 



camera drawing from a permanent preparation shown in Fig. 125 throws consider- 

 able doubt upon this interpretation of the first three division planes as described by Stein. 

 According to this one preparation the third division is horizontal, giving four cells above and 

 four below. The plate form is assumed in the early sixteen-cell stage. 



