THE RHOMBOZOA 



rounded by the products of the micromeres (Fig. V.) ; then the macromere 



divides into two unequal cells ; the larger again divides in the same way ; 



so that the central mass at this stage consists of an axial cell with two 



small " primary germ cells," embedded in its cytoplasm one at each end. 



These primary germ cells are comparable 



to the "intermediate cells" of Ortho- 



nectida, but have a different fate. Here, 



in the Dicyemida, there is an early 



setting aside of cells for reproduction, just 



as there is in Ascaris and other higher 



forms. The axial cell remains apparently 



quiescent throughout life, but is possibly 



engaged in elaborating nutritious juices 



absorbed from the host by the ectoderm 



cells; it may be considered as an endo- 



derm cell the last representative of a 



degenerate enteron. But the primary 



germ cells soon divide, each into two, and 



then into four ; so that there are eight 



germ cells lying within the axial cell. 



In some individuals (known as " ne- 

 matogens ") each germ cell proceeds to 

 undergo segmentation, and develops into 

 a " vermiform embryo " of a comparatively 

 simple form a miniature of the parent 

 (Fig. V.). But in other individuals, 

 known as " rhombogens " differing, ac- 

 cording to Whitman, in no definite way 

 from the former the germ cells pass 

 through a very peculiar series of stages, 

 and each produces a number of " infusori- 

 forrn embryos." These two kinds of 



embryo were noted by Erdl, who regarded suit of segmentation is a solid biasto- 

 them as two different stages in the life- SS^vS?? ^SS^SS^oi 



history. Kolliker was the first to recog- ectoderm cells (a); from the macromere 



', . _. i . a primary germ cell (</) has been given 



nise that they are dimorphic forms ; off. In 2, 3, the embryo has elongated, 



whilp v "Rpnprlpn anrl lafpr Whitman tlie ectoderm is ciliated and already 



. v. tfeneden, and later, Wnitman, differeht i a ted into cap cells (d) and 



more accurately traced OUt their develop- body cells. The germ cell has divided 

 mi /* i into two. and t!i6S6 luivc sunk into tli6 



ment. The vermiform embryos escape substance of the axial cell, in 4 the 



from the nematogen and swim about with vermiform embryo has put on all the 



. 5 . . characteristics of a Dicyema; each of 



their parents 111 the renal sac of the host, the two germ cells has already sub- 



whirh tViPVTiPvpr Ipavp thpv arP inrlpprl divided to form four, of which one 



wmcn tney never leave , tney are, indeed, above and two below are procee din g to 



killed by sea- water. They grow into develop into a second generation, c, 



nematogens, and repeat the history of n 

 their parent 



But the infusoriform embryo is a more complicated organism (Fig. VI.). 

 It is nearly spherical, and built up of the following parts : (1) Of an 

 hemispherical cup of ten ciliated cells; (2) of two large cells containing 

 refringent bodies, and occupying the " anterior " part of the surface of the 



FIO. V. 



Four stages in the development of 

 Diciiema 

 In 1 tl 



