THE GREGARINES AND COCCIDIA 



333 



The generative chromatin may organize itself into a definite secondary nucleus 

 (" micronucleus ") during the break-up of the sporont-nucleus, as in Ptero- 

 cephalus (Fig. 145) ; or the first spindle arises within the sporont-nucleus 

 before it breaks up, as in G. ovata (Fig. 146) ; or a number of distinct chromo- 

 somes are formed in the sporont-nucleus during the process of its disintegration, 

 which pass to the exterior of the nucleus and form the equatorial plate of a 

 spindle of which the achromatinic elements appear to arise chiefly outside the 

 nucleus, as in Monocystis rostrata. In either case the first spindle consists 

 only of the generative chromatin ; the remainder of the original sporont-nucleus 

 is disintegrated and absorbed, or is left over in the residual protoplasm of the 

 cyst. The statement of Kuschakewitsch, to the effect that the primary 

 nucleus of the sporont may break up into a mass of chromidia, from which 

 a number of secondary (generative) nuclei are re-formed, has not received 

 confirmation in any quarter. 



The mitoses in the sporont are remarkable, in most cases, for the very 

 distinct centrosomes (Fig. 147), which appear at the side of the nucleus before 



FIG. 146. Two stages in the formation of the first division-spindle of Gregarina 

 ovata, showing its origin from a very small part of the primary nucleus. In 

 A the spindle is seen within the primary nucleus ; in R the spindle is becoming 

 free from it at one point, after which the remainder of the primary nucleus 

 degenerates. After Schnitzler ; magnification 850 diameters. 



division begins as a grain or a pair of grains placed at the apex of a " cone 

 of attraction" ; in Monocystis rostrata, however, centrosomes appear to be 

 absent. The number of chromosomes in the equatorial plate is usually four ; 

 but in Monocystis rostrata the number appears to be eight, and in Pterocephalus 

 and the allied genus Echinomera there are five chromosomes, four of ordinary 

 size and one large unpaired chromosome. Unlike the unpaired chromosome 

 of Metazoa, that of the gregarines is present in both sexes ; it gives rise, during 

 the reconstitution of the daughter nucleus, to the karyosome ; and the karyo- 

 some is eliminated from the nuclear spindle at the subsequent mitosis. 

 The significance of the unpaired chromosome is far from clear, and requires 

 further elucidation. 



5. Each of the nuclei of the preceding stage grows out from the 

 surface of the body surrounded by a small quantity of protoplasm, 

 and thus a great number of small cells are budded off over the 



