THE EARLY DEVELOPMENT OF AMPHIOXUS. 605 



4. The Origin of the Atrial Folds. 



Kowalevsky (7) was the first to discover that the atrial 

 cavity was formed by the meeting in the mid-ventral line of 

 two long ridges or folds. These, which were more exactly 

 investigated by Lankester and Willey (9), are situated at the 

 ventro-lateral angles of the body, and the atrial cavity is at 

 first a small space situated in the middle line beneath the 

 pharynx (fig. 23, b). Later the atrial cavity extends up at 

 the sides of the pharynx, and the origin of the folds becomes 

 consequently shifted up the body. This is the account of the 

 origin of the atrial cavity given by Lankester and Willey ; but 

 it must be remembered that as the dorsal limits of the atrial 

 cavity are from the beginning conterminous with those of the 

 gill-slits, the process might be more correctly described as a 

 great relative growth of the ventral region of the pharynx and 

 surrounding structures. Lankester (8) terms the folds which 

 actually wall in the atrial cavity '' epipleural,^' and the pro- 

 jecting angles after these folds have united " metapleural." 

 I shall use the term "atrial fold" to include the whole, of 

 which both are parts. 



It must already have struck the reader that the posterior 

 ventral extensions of the collar cavities which I have described 

 above occupied precisely the region where the atrial folds 

 subsequently appeared ; hence it will not be surprising when 

 I state that the cavity of the atrial fold, termed by Lankester 

 and Willey " pseudocoelic,'' is nothing but the backward ex- 

 tension of the collar cavity. This I have succeeded in proving 

 for the right collar cavity (comp. fig. 22, a, b, and c) ; and since 

 the left collar cavity has precisely the course which Willey 

 describes for the oral hood and left atrial fold, no one will 

 doubt that this is the case for the left side also. From the 

 walls of these two collar cavities the ventral muscles of 

 Amphioxus are formed, and their lumen becomes occluded in 

 the centre (fig. 24), but remains at the sides as the "meta- 

 pleural lymph canal." 



Lankester and Willey describe the atrial folds as first 



