DEVELOPMENT OF A MYXOSPORIDIAN 339 



each has a very different later history. One becomes the vege- 

 tative nucleus and, in most cases, undergoes no further division, 

 while the other divides a number of times to form the spores. 

 At a little later stage each trophozoite contains three nuclei, 

 one being considerably larger than the other two (figs. 16 to 21). 

 The two smaller nuclei are derived from one of the nuclei of the 

 preceding stage by mitotic division while the other nucleus re- 

 mains undivided. The latter gradually increases in size, the 

 specially differentiated area of surrounding cytoplasm becoming 

 indistinguishable from the rest of the trophozoite. This forms 

 the vegetative nucleus and ordinarily undergoes no further 

 changes, but rarely may, in later stages, divide amitotically 

 (fig. 30). This division of the vegetative nucleus is very ex- 

 ceptional and only in rare cases have two vegetative nuclei been 

 found in the same trophozoite. The two smaller nuclei are the 

 generative nuclei and soon divide by mitosis to form four nuclei 

 of equal size (figs. 23 and 24). One of the generative nuclei 

 usually divides before the other (figs. 20 to 22) so that tropho- 

 zoites with but four nuclei are common. As a result of these 

 divisions trophozoites are formed with five nuclei, one larger 

 vegetative nucleus, and four generative nuclei of equal size, each 

 surrounded by a denser more deeply staining area of cytoplasm. 

 The generative nuclei then become grouped in pairs (fig. 24), 

 each pair forming a sporoblast. At the same time one of the 

 nuclei of each pair divides mitotically, resulting in sporoblasts 

 with three nuclei, one nucleus in each being considerably larger 

 than the other two (figs. 25, 27, 28, 29 and 32). Trophozoites 

 with two sporoblasts, each with three nuclei, are more abundant 

 in my preparations than any other stage. At this time the cyto- 

 plasm immediately surrounding the sporoblast nuclei is much 

 denser and more clearly defined than at any preceding stage, 

 and is sharply marked off from the surrounding cytoplasm which 

 now stains less deeply than before. Each sporoblast is, I be- 

 lieve, composed of three distinct cells corresponding to the 

 three nuclei, but in many cases the cells are so closely associated 

 that they can only be distinguished with difficulty, and some- 

 times not at all. In all cases during this and later stages the 



JOURNAL OF MORPHOLOGY, VOL. 27, NO. 2 



