304 THE SPOROZOA 



almost entirely of numerous pansporoblasts (Fig. 1 1 8, b and c), which 

 are continually being formed at the two poles of the trophozoite. 

 Between the pansporoblasts, septa or partitions extend in from 

 the inner coat, isolating them from one another, so that the entire 

 endoplasm has a chambered or alveolar structure, each chamber 

 containing originally a single pansporoblast (Fig. 119). Towards 

 the centre of the body each pansporoblast has developed into a mass 

 of spores or other germs, completely filling a chamber (Fig. 120); 

 towards the poles pansporoblasts or early stages in spore-formation 

 are found, and the two extremities of the body are occupied by 

 the two regions of proliferation, so that the parasite grows by 

 forming new spores at its two ends. 1 



In the third stage the parasite is encysted in the connective 

 tissue, as above described. The body -form is less elongated, 

 having become more or less rounded off, and the two polar areas 

 of proliferation now extend round the whole trophozoite, forming 

 a peripheral zone, the external layer of the body of the parasite, 

 consisting of small alveoli, containing elements in process of 

 development. Internally to the zone of proliferation are found 

 alveoli crammed with ripe spores, which constitute an inter- 

 mediate zone. The centre of the body is occupied by a granular 

 substance, derived from disintegration of the centrally placed 

 spores, which having become stale and past their prime, die and 

 break up. The parasite continues to grow within the cyst, new 

 spores being formed towards the periphery, to replace those which 

 die off towards the centre. The development of the spores has 

 not been followed in detail, but the pansporoblasts have at first 

 one nucleus, later several. 



The germs or reproductive bodies which arise from the pan- 

 sporoblasts within the alveoli appear to be of at least two kinds, 

 which may be conveniently distinguished as chlamydospores and 

 gymnospores respectively. 



The chlamydospores, commonly termed " spores " simply, are 

 minute rod -like bodies, which vary in details of form and size 

 in different species. In Sarcocystis tenella of the sheep (Fig. 122) 

 they measure about 14 /x in length by 3 /x in breadth. One 

 extremity is more rounded, the other pointed. The spore- 

 membrane is very thin, and the chlamydospores themselves very 

 delicate and fragile ; they are easily acted upon by reagents, 



1 Smith [121], in his recent account of the formation of gymnospores in Sarcocystis 

 muris, gives a different interpretation of the various stages. He terms " sporoblasts " 

 the " roundish or polyedral masses " which have been homologised above with pan- 

 sporoblasts, and considers that the partitions between them are not ingrowths of the 

 internal wall, but are " simply the walls of the sporocysts in close apposition with one 

 another." He regards the gymnospores as sporozoites. Smith's terminology is the 

 outcome of a misleading comparison of the Sarcosporidia with the Coccidia. He does 

 not seem to have seen true spores. 



