110 



INFUSORIFORM EMBRYO. 



directed forwards, and in its edge are imbedded a number of rod-like 

 corpuscles which appear as a ring near the surface in a full-face view 

 (fig. D). The lid has the form of a low pyramid with its apex directed 

 outwards. It is made up of four segments (fig. D). The contents 

 of the urn, which completely fill up its cavity, are four polynuclear 

 cells arranged in the form of a cross which appear with low powers 

 as granular bodies (fig. F). They are frequently ejected, apparently 

 at the will of the embryo. 



The refractive bodies (r), two in number, one on each side of the 

 middle line, are composed of a material which is not of a fatty nature, 

 and which is passive to the majority of reagents. Each is enveloped 

 in a special capsule, and at times more than one refractive body is 

 present in each capsule. The tail is a conical structure formed of 

 ciliated granular cells. 



No plausible guess has been made as to the function either of the 

 urn or of the refractive bodies. 



The infusoriform embryos originate from germs, which have 

 however a different origin to the germs of the vermiform embryos. 

 One to five cells appear in the axial hypoblast cell, in a way not clearly 

 made out, and each of them gives rise by an endogenous process 

 to several generations of cells, all of which develope 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. 



u. wall of urn; I. lid of urn; 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 formation. 

 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 concentric layers, 



