DEVELOPMENT OP THE ATRIAL CHAMBER IN AMPHIOXUS. 649 



So far from confirming the existence of "folds," or admitting 

 anything which Professor MacBride can justifiably term 

 "atrial folds/^ we showed that there are no atrial 

 folds. We showed that what Kowalevsky had mistaken for 

 "atrial folds" are really the metapleura, and that these do 

 not grow round and meet in the middle line, but that a very 

 small in-sinking is formed between them, and is covered in 

 by a minute horizontal growth right and left which we called 

 the " subatrial ridges or folds/' their union resulting in the 

 formation of what is, at first, a very narrow "subatrial floor" 

 lying between the two upstanding metapleura. "We showed, 

 then, that "the two long folds 'of Kowjikvsky, qi Gl(d by 

 MacBride as though it were established that they are the 

 rudiments of the epipleura (as in Bolph's scheme) do not form 

 the atfial cavity, but are really the metapleura. MacBride says 

 we confirmed Kowalevsky, and that Kowalevsky "discovered/' 

 the mode of formation of the atrial cavity. He did not do so, 

 but, on the contrary, was misled as to the nature of what he 

 thought to be coalescing folds. And Willey and I did not 

 confirm him, but discovered a totally diflPerent mode of forma- 

 tion. 



As to the continuity of the cavity of the metapleura with the 

 coelom, Willey and I showed that the space in the metapleura 

 is not at any time freely open above into the adjacent coelomic 

 cavity, as was figured by Kowalevsky, who mistook the meta- 

 pleural cavity for the lateral division of the coelom, in which 

 the gonads develop. We reproduced Kowalevsky's figure, and 

 showed that it was defective. MacBride confirms us in deny- 

 ing a continuity of the lymph-space of the metapleur with the 

 adjacent coelom. We suggested, with reserve and caution, 

 that the lymph-space of the metapleur might form as "pseudo- 

 coel," — that is, as an intercellular space, — having no relation 

 to enteric pouches. MacBride has now shown that the most 

 anterior pair of enteric pouches (the collar-pouches) are the 

 sources of the lymph-spaces in the metapleura. This is cer- 

 tainly altogether a different relation to that indicated by 

 Kowalevsky's transverse section (fig. 1, p. 449, of Willey's 



