134 



DICYEMIDyE. 



a way not clearly made out, and each of them gives rise by an 

 endogenous process to several generations of cells, all of which 

 develop into infusoriform embryos. 



FIG. 62. INFUSORIFORM EMBRYO OF DICYEMA. 



A. B. C. Three of the later stages in the development. 



D. E. F. Three different views of the full-grown larva. D. from the front, E. 

 from the side, and F. from above. 



G. side view of urn. 



. wall of urn ; /. lid of um ; r. refractive bodies ; gr. granular bodies filling the 

 interior of the urn. 



The primitive cell is called by Van Beneden a Germogen. 

 In its protoplasm a number of germs first appear endogenously, 

 but the nucleus of the germogen does not assist in their forma- 

 tion. They eventually become detached from the parent cell, 

 around which they are concentrically arranged. A second and 

 then a third generation of germs are formed in the same way, till 

 the whole of the protoplasm of the primitive cell is absorbed in 

 the formation of these germs, and nothing of it remains but the 

 nucleus. The germs so formed are arranged in about three con- 

 centric layers, of which the innermost is the youngest. One to 

 five masses of germs may be present in a single Rhombogen. 

 The germs undergo a division, in the course of which their nuclei 

 exhibit very beautifully a spindle modification. In the course of 

 the segmentation the embryo gradually assumes its permanent 

 form, and four of the cells composing it can be distinguished 

 from the remainder by their greater size (fig. 62 A, ). The two 

 largest of these give rise to the wall of the urn, and also give 

 origin to four smaller cells (fig. 62 B, gr) which eventually be- 

 come polynuclear and constitute the four granular cells in the 

 urn. The two other cells become the lid of the urn. The parts 



