1918.] T. Southwell & B. Peashad : Indian Fish Parasites. 345 



The wall of the cyst is formed of a vertically striated portion showing 

 no nuclei. This part takes up the cytoplasmic eosin-stain only. It is 

 covered externally by an epithelium two to three layers thick. Internal 

 to the vertically striated portion of the cyst there is the endoplasmic 

 layer. The layer shows a coarse granular structure ; the nuclei, which 

 lie scattered in the cytoplasm, are either round or elongated, and 

 measure 3 to 5 /x in length. No cell limits could, however, be dis- 

 covered. 



In all the cysts examined, spore formation had already proceeded 

 to an advanced stage, and it was clear that the whole of the cavity in 

 the middle of the cyst was occupied by mature spores, whilst the pan- 

 sporoblasts and the immature spores were lying round the periphery. 

 In many cases these pan-sporoblasts were seen lying amongst the nuclei 

 of the endoplasm. 



Formation of the spores. — In the endoplasm two sorts of nuclei are 

 to be seen, viz., the vegetative and the generative. The latter always 

 occur in rounded cells, which have been variously designated as " spheres 

 primitives " by Thelohan, " pan-sporoblasts " by Gurley, and " Propa- 

 gation Zellen " by Keysselitz. They are, as has been described above, 

 of a rounded shape, with a marginally situated nucleus (figs. 2-4). These 

 cells vary in size from 6 to 11/^, and, in the nucleus, a distinct caryosom 

 can easily be distinguished. The pan-sporoblasts were in all cases 

 seen to divide mitotically into two (figs. 5-9). In this particular 

 the present species differs from Myxobolus toyamai, Kudo, where the 

 pan-sporoblasts divide into two or three daughter cells. In the division 

 of the pan-sporoblasts, the chromatin was seen first to form a coiled 

 thread, which later on splits up by mitosis into two parts, one for each 

 of the daughter cells. The next stage that occurs is one in which two 

 cells are united together (fig. 10). These cells are the micro- and macro- 

 gametes, a fact which is evident from the size of the cells. No separate 

 micro- or macro-gametes were found. The next stage in the union of 

 these two cells is the formation of a sinuous chromation thread in the 

 nucleus of the macro-gamete (fig. 12). The nuclear membrane of the 

 micro-gamete was next seen to disappear. At the same time the nucleus 

 of the macro-gamete showed a chromatin thread, and in the next stage 

 two chromatin spiral threads were to be seen, lying in the cytoplasms 

 of the micro- and macro-gametes respectively. Each of these chromatin 

 threads splits up into two, and four nuclei, two large and two small, 

 are formed in the united cystoplasmic substance of the micro- and macro- 

 gametes (figs. 13-17). In our preparations the next stage seen is a 

 pan-sporoblast cell with five fully formed nuclei, surrounded by definitely 

 marked cystoplasmic areas, and a large chromatin mass lying free (fig. 

 18). In the final stage ten fully formed nuclei can be distinguished 

 in the mother pan-sporoblast, besides two nuclei for the pan-sporoblast 

 mother cell, and reduction nuclear chromatin particles lying free in the 

 cystoplasm of the mother cell. Intermediate stages are also present. 

 The pan-sporoblast next divides into two daughter cells or sporoblasts, 

 each with five nuclei ; two of these unite later on to form the nucleus 

 of the sporoplasm portion of the spore, the one which is seen lying close 

 to a vacuole forms the nucleus for the polar capsule and the other two 



