THE NEOSPQRIDIA 411 



The binucleate amoebula is succeeded by a stage with four cells (Fig. 170, C), 

 the result of the division of each nucleus, with subsequent division of the 

 cytoplasm of the amoebula. Two of the cells take up a superficial position 

 and form an envelope for the other two, which are the gametocytes. The 

 two enveloping cells do not develop further, but the two internal cells proceed 

 to multiply by repeated division to form sixteen gametes (Fig. 170, D, E), eight 

 derived from each of the gametocytes. The gametes now copulate in pairs, 

 those of each couple being slightly different from one another, chiefly as 

 regards the size of their nuclei. It is very probable that in each couple one 

 gamete is descended from one of the two original gametocytes, the other 

 from the other (Fig. 170, F, G). In this way eight zygotes are formed, each 

 of which represents a sporoblast and proceeds to form a spore. 



Each sporoblast now divides into two cells, which may be distinguished 

 as Cell A and Cell B respectively. Cell A is the mother- cell of all the accessory 

 elements of the spore namely, parietal cells and capsulogenous cells. Cell B 

 is the mother-cell of the germinal elements. The development of these two 

 sets of elements proceeds at first quite independently. Cell A divides into 

 six cells (Fig. 170, H, /, right), three parietal cells which secrete the three 

 valves of the sporocyst, and three capsulogenous cells which produce the 

 three polar capsules. Cell B is at first a cell with a single large nucleus, which 

 now begins to divide, and when it does so Cell B separates from the six cells 

 derived from Cell A (Fig. 170, H, I, left). As a result of the nuclear division 

 in Cell B, it becomes a large multinucleate plasmodium, the germinal mass, 

 containing larger central nuclei, and smaller towards the periphery. The 

 larger nuclei are perhaps trophic in function, the smaller germinal. 



As a result of these changes, the body now consists of two envelope-cells, 

 destined to degenerate, containing sixteen cell-masses ; eight, consisting each 

 of the six spore-forming cells, which take up a more central position, and 

 eight multinucleate germinal masses, which lie at the periphery of the body. 

 Each central mass forms the sporocyst and polar capsules of the spore, and 

 when these parts are completely formed the germinal masses migrate bodily 

 into the spores, each germinal mass occupying the cavity of one of the spores 

 (Fig. 170, J). Within the spore the germinal mass remains for a time in the 

 condition of a multinucleate plasmodium, but divides ultimaely into a vast 

 number of uninucleate sporozoites. The spore germinates, doubtless, in the 

 digestive tract of a new host, setting free a swarm of amosbulae which as 

 planonts pass through the intestinal epithelium and initiate a fresh develop- 

 mental cycle. 



The spore-formation in Actinomyxidia is seen to agree in all 

 essential details with that of the Myxosporidia, and inasmuch as 

 each zygote becomes a sporoblast, and gives rise to an entire spore, 

 with all its accessories, the process is similar to that of the Disporea. 

 The chief points in which the Actinomyxidia differ from the Myxo- 

 sporidia are the absence of the large trophic plasmodial stage, the 

 ternary symmetry of the spore, and the enormous number of sporo- 

 zoites contained in the relatively huge spore. 



Order III\ : Microsporidia. The characteristic feature of this 

 order is furnished by the spores, which are minute oval refringent 

 bodies in which no polar capsule is visible in the fresh condition ; 

 but when treated with reagents the spores are seen to contain, 

 with one exception, a single polar capsule, from which, after suit- 

 able stimulation, a polar filament of very great length is extruded. 

 The existence of the polar capsule in the Microsporidian spore was 

 discovered by Thelohan, who in consequence of this discovery 



