66 



PROTOZOA. 



still continues to grow and to move. When a spore is about to lie formed, a 

 small spherical mass of endoplasm containing one nucleus is marked off from 

 the rest by a delicate membrane ; it thus constitutes an ovum-like body lying 

 in the endoplasm. It is called a primitive, sphere (Fig. 55) ; its nucleus gradually 

 divides, karyokinetically, into ten nuclei, and it then itself divides into two parts, 

 each of which contains three of Ihe ten nuclei of the primitive sphere ; the four 

 remaining nuclei together with a portion of protoplasm forms a small residual 

 body, which soon disappears (cf. the residual bodies of other Sporozoa). The 

 two trinucleated bodies thus formed are the sporoblasts ; they are enclosed in 

 the membrane of the original primitive sphere, which soon thickens into a 

 resistent spore-case. Each sporoblast divides into three cells (Fig. 55e), of 

 which one gives rise to one spore, and the other two to the two polar-capsules 

 (Fig. 55/). The polar-capsules are formed in and at the expense of the protoplasm 

 of the polar-capsule cells, which wholly disappear in the process. The polar- 

 capsules are ovoid bodies containing a long spirally-coiled thread (Fig. 56), which 

 is everted with considerable force when the spore is acted upon by the digestive 



a 



d 



FIG. 55. Spore-formation of Myxdbolus (after Thelohan). a, primitive sphere with nucleus, 

 a little endoplasm of the parent is shown ; 6, stage with six nuclei, and c, with ten nuclei. 

 d, division of primitive sphere into two sporoblasts a and b, each with three nuclei, and a 

 residual body with four nuclei n ; e, division of one of the sporoblasts into two smaller 

 capsule-forming cells cp, each with a vacuole, and a larger cell (the spore) g. f, formation 

 of polar-capsules. 



juices of the animal which swallows it. It is probably everted with such force 

 that it pierces the wall of the alimentary canal, and thus effects the attachment 

 of the spore to its new host (Fig. 56). The spore-case bursts in the course of 

 twenty-four hours after this attachment, and the contained germ makes its 

 way as an amoeboid form through the intestinal wall and migrates to the tissue 

 in which it is to live. 



In the case just described, which is that of the genus Myxobolus, each primitive 

 sphere gives rise to two spores ; but in some cases only one spore results, and in 

 others three or more are formed. In the tissue-forms a considerable number of 

 spores proceed from each primitive sphere. 



The species are distributed by the spores, which are carried to the exterior, 

 or when this is impossible, as in the case of the tissue-forms, are set free on the 

 death of their host. 



