ANATOMY AND DEVELOPMENT. 209 



diate intention of treading. The pineal eye may not have 

 been primitively so much an organ of vision as a light- 

 perceiving organ, as is no doubt the case with the eye of 

 the Tunicate tadpole. 



We may at least conclude that there can be no doubt 

 that the Tunicate eye is the functional homologue of the 

 pineal eye of the higher Vertebrates, as Spencer sug- 

 gested. 



Stomodceal and A trial Involutions. 



By the time that the cerebral vesicle of the Ascidian 

 embryo with its contained sense-organs (eye and otocyst) 

 is approaching the completion of its full development, no 

 less than three ectodermic invaginations occur in the body 

 of the embryo. 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 subjacent endoderm. This is the stomodceum, 

 and its formation is preliminary to the perforation of the 

 mouth which takes place later, and places the stomodosum 

 in open communication with the portion of the enteric 

 cavity which will become the branchial sac (Fig. 102). It 

 should be emphatically noted that the stomodoeal invagi- 

 nation occurs in the dorsal middle line immediately adja- 

 cent to the anterior extremity of the central nervous 

 system. 



The other two ectodermic invaginations occur symmetri- 

 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 atrial involutions, which, by their 

 subsequent growth and modification, give rise to the atrial 

 or peribranchial cavity. We see, therefore, that the epi- 

 thelium which forms the lining membrane of this cavity 

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



