34 



THE PINEAL ORGAN 



in general form between the Cephalo-thoracopagus and the Diprosopus 

 types, the principles which are concerned in the development of the cy clops 

 eye belong to the same category, although in the one case (Fig. 23, B) 



R - L 



(R+L) 





Fig. 26. 



A — The dorsal halves Y and Z of two 

 Triton gastrulas grafted together so 

 that the directions of invagination 

 at their blastopores are directly 

 opposed to each other. 



B — The resulting embryo showing 

 crossed doubling — duplicitas 

 cruciata — each half gastrula has 

 developed a single posterior trunk 

 region with spinal cord and two 

 "secondary" head regions. These 

 are formed at right angles to the 

 longitudinal axis of the trunks. 



L R, region which will give rise to the 



\ ^ L and R eyes of one "secondary" 



face; (R+L) region which will 



give rise to the cyclops eye of the 



other secondary face. 



Fig. 27. 



C 1 — A later stage of the duplicitas 

 cruciata larva, viewed from the 

 direction of the arrows L R seen 

 above the drawing in B, Fig. 26. 



In C- the same larva is viewed from 

 the direction of the single arrow 

 (R-tL) in B, Fig. 26, which points 

 to the region which will give rise to 

 the cyclops eye, seen in C 2 . In 

 both C and C 2 the trunk and tail 

 regions are viewed from the side. 

 The two " secondary faces " are 

 developed so that each is directed 

 at a right angle away from the 

 dorso-ventral plane of the bodies 

 of the larvae. 



Z : the component larvae, 

 dorsal. V : ventral. 



D : 



(After Spemann.) 



the cyclops eye is formed by the union of the temporal halves of the 

 opposed eyes and in the other (Fig. 25), by the union of the nasal halves 

 of the two opposed eyes. In both instances the segments of the developing 



