ciate intention of treading. The oineal eve mav not have 

 hee" primitively so much an organ of \hsion as a ii^^h:- 

 percei\ing organ, as is no doubt the case wdth the eve of 

 the Tunicate tadpole. 



\\'e mav at least conclude that there can be no doubt 

 :hat the Tunicate eve is the functional homologue of the 

 pineal eye of the higher Venebrates. as Spencer sug- 

 gested. 



S:::?>::d^j: jkJ ArHal Im-cl-itions. 



Bv the time that the cerebral vesicle of the Ascidian 

 embr\"o with its contained sense-organs ^eve and otocyst) 

 is approaching the completion of its full development, no 

 less than three ectodermic invaginations occur in the body 

 of the embr^'o. One of these is situated immediately in 

 front of and in contact with the anterior wall of the cere- 

 bral vesicle, the blind end of the involution pressing 

 against the subiacent endoderm. This is the s:s-!noJ<xuf!i, 

 and its formation is preliminan' to the perforation of the 

 mouth which takes place later, and places the stomodceum 

 in open communication with the portion of the enteric 

 ca-^dtv which will become the branchial sac iFig. ion. It 

 should be emphatically noted that the stomodcEal invagi- 

 nation occurs in the dorsal middle line immediatelv adja- 

 cent to the anterior extremits" of the central ner\"0us 

 svstem. 



The other two ectodermic invaginations occur s\Tnmetri- 

 cally, one to the right and the other to the left of the 

 dorsal middle line, behind the region of the cerebral vesicle, 

 and constitute the pair of atria! iui-obitiois. which, bv their 

 subsequent growth and modification, give rise to the atrial 

 or peribranchial ca\-ity. We see. therefore, that the epi- 

 theKum which forms the lining membrane of this ca\dtv 

 is, as in Amphioxus. derived from the external ectoderm. 



