THE EYES OF UROCHORDA 



175 



eyes of vertebrates, but was comparable with the parietal eye of verte- 

 brates. Froriep, however, considered that the site of origin of the 

 primary optic plate in the early stages of development of the ascidian 

 larva, namely on the right side of the cerebral vesicle, and the appearance 

 of a similar but more rudimentary optic organ on the left side were 

 presumptive evidence of the organs being homologous with the paired 

 lateral eyes of vertebrates rather than with the " unpaired " median 



Fig. 127. — Head-end of an Ascidian Embryo, Phallusia mammillata, seen 

 from the Dorsal Aspect. The Epithelium of the Wall of the Cerebral 

 Vesicle on the Right Side is modified so as to form a Retinal or Optic 

 Plate, consisting of Radiating Columnar Cells, the Inner Ends of 

 which are Pigmented. 



adh.p. : adhesive papilla. 



c. can. : central canal of medullary tube. 



cl. v. : " cloacal vesicle " or atrium. 



int. : intestine. 



n.t. : wall of medullary tube. 



ot. : statocyst. 



p. : pigment. 



r. eye : right eye. 



s.v. : interior of sensory vesicle. 



Compare with Fig. 3, which represents an older stage of development. 

 (From Korschelt and Heider, after Kowalevsky.) 



or parietal eye of vertebrate animals. In this connection it is important 

 to note that the statocyst of the ascidian larva is also a single organ, 

 and that in explanation of this it seems likely that the full development 

 of the corresponding area on the opposite side of the head and brain 

 to that on which the existing statocyst has been developed was curtailed 

 or arrested at an early stage. On the other hand, at the time (1906) when 

 Froriep was writing it was not generally appreciated that the " parietal 

 eye " of vertebrates has been derived from paired median organs of 



