220 ARTHUE WILLEY. 



with the region of the first myotome, is held by van Beneclen 

 and Julin to be homologous with the prsechordal portion of the 

 alimentary canal in the Ascidian tadpole. In the latter case, 

 also, two horn-like outgrowths are developed from this part of 

 the enteric cavity. They are subsequently met by two inva- 

 ginations — right and left — of the epidermis, with which they 

 fuse to form the so-called primary branchial canals. They 

 become the right and left peribranchial cavities, and consist 

 partly of epiblast and partly of hypoblast, and are considered 

 to be homologous with the right and left anterior intestinal 

 diverticula of Amphioxus, in which, however, the epiblastic 

 element is wanting. 



The peribranchial cavities then expand, and, growing round 

 dorsally, fuse together to form the atrium. Meanwhile the 

 characteristic gill- slits or stigmata have commenced to form, 

 leading from the branchial to the peribranchial cavity, and 

 finally the left portion of the latter cavity alone communicates 

 to the exterior by the atriopore, the orifice of invagination of 

 the right portion of the cavity having been lost. It is stated 

 that probably the original communication of the primary 

 branchial canals with the enteric cavity is lost, and that all the 

 stigmata proper are secondary. 



Thus the visceral wall of the Ascidian atrium is derived mainly 

 from the hypoblast, while the peripheral wall is formed from the 

 epiblast, and therefore the part of the atrium which leads to 

 the exterior on the left side by means of the atriopore consists 

 of epiblast j so that the prseoral pit of Amphioxus, which is 

 composed entirely of hypoblast, does not exactly correspond to 

 the atrium of Ascidians, but is represented in the latter 

 group by the visceral wall of the atrium, while the actual 

 opening to the exterior of the prseoral pit corresponds to the 

 junction of the hypoblastic and epiblastic elements in the 

 Ascidian atrium. 



Neglecting their subsequent modifications, and supposing 

 the epiblastic involutions to have been suppressed or reduced 

 to zero in Amphioxus, the anterior intestinal diverticula of 

 the embryo of the latter do actually coincide in every essential 



