INFUSORIFORM EMBRYOS. 133 



At the time of birth the embryo may contain a number of germs 

 and sometimes even developing embryos. 



Infusoriform Embryos. The infusoriform embryos are 

 capable of living in sea-water and almost certainly lead a free 

 existence. In their most fully developed condition so far known 

 they have the following rather complicated structure (fig. 62 D, 

 E, F, G). 



The body is somewhat pyriform, with a blunt extremity 

 which is directed forwards in swimming, and a more pointed 

 extremity directed backwards. The former may be spoken of as 

 the anterior, and the latter as the posterior extremity or tail. 

 At the anterior extremity are situated a pair of refractive bodies 

 (r) which lie above an unpaired organ which may be called the 

 urn. 



The structure of the urn, the refractive bodies, and the tail 

 may be dealt with in succession. 



The urn consists of three parts: (i) a wall (ft), (2) a lid (/), 

 and (3) contents (gr). The wall of the urn is hemispherical in 

 form, and composed of two halves in apposition (fig. F). Its 

 concavity is 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 com- 

 pletely 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 



