INTERNAL ANATOMY. 



23 



frog-tadpole the fold of one side becomes continuous ven- 

 trally with that of the other, so that in effect we have one 

 large semicircular fold covering over the gill-slits. Event- 

 ually the hinder free margin of the fold undergoes con- 

 crescence with the body-wall, so that a single peribranchial 

 cavity is formed about the gills. This cavity is closed all 

 round except at one point, usually on the left side, but 

 sometimes in the mid-ventral line, where it remains open 

 as the poms brancldalis, or so-called spiracidum. 



This comparison of Hux- 

 ley's was extremely well 

 taken, and although the two 

 cavities, namely, the peri- 

 branchial cavity of the frog- 

 larva and the atrial chamber 

 of Amphioxus, are probably 

 by no means homologous, or 

 genetically related to each 

 other, still the close analogy 

 that exists between them is 

 most instructive, and yet, 

 singular to say, it did not 

 lead Huxley to a correct 

 interpretation of the atrial 

 chamber.^ 



Its true nature was at 

 length established by Rolph 



in 1876. By comparing his own observations on the adult 

 with those of Kowalevsky on the larva, Rolph came to the 

 conclusion that the atrial cavity of Amphioxus originated 

 by the growth of two folds of the body-wall over the gill- 

 slits on each side, and by their subsequent fusion in the 

 mid-ventral line except at one point, which remained open 



Fig. 8. — Tadpole of Frog {Rana cla- 

 mata) from ventral side. (Original.) 



cl. Dextrally placed cloacal aperture. 

 m. Mouth, sp. spiraculum ; the dotted 

 line indicates the extent of the opercular 

 chamber. /. Tail. 



