DEVELOPMENT OF ATRIAL CHAMBER OP AMPHIOXUS. 451 



There is no evidence that this space is an offshoot of the 

 original myocojlomic pouches : it arises apparently as an inter- 

 cellular space in the midst of the connective tissue ; in fact, it 

 would seem to belong to that category of spaces to which the 

 term "pseudocoel" has been applied. If this should prove to 

 be its history it would stand in contrast to the spaces in con- 

 nection with the dorsal and ventral fins, which have been 

 shown by Hatschek to be derived directly from the myo- 

 coelomic pouches. 



Rolph's figures (Figs. 3, 4, 5) do not profess to be more than 

 diagrams. They show the epipleur originating as a depending 

 ridge on each side of the pharynx (Fig. 4). Into this ridge the 

 coelom is extended. The epipleura meet finally in the middle 

 line below the pharynx according to this theory (Fig. 5). It is 

 no doubt true that the scheme of growth thus sketched by 

 Rolph, and based upon Kowalevsky^s erroneous figures^ would 

 account satisfactorily for the condition of the atrial chamber 

 and its epipleural walls, as observed in the adult. It also gives 

 a basis for the suggestion made by Kowalevsky that the 

 epipleura are comparable to the opercula of Teleostean fish. 



We shall now give an account of our recent observations. 



Formation of the Atrial Chamber as now 

 determined. 



The first indication of the commencing formation of the 

 atrial chamber is to be found in larvae with nine or ten gill-slits 

 on the right side. Behind the region of the pharynx we find 

 that the mid-line of the body has become marked with a narrow 

 groove, so that in section it is bifid (Fig. 6). The short up- 

 standing ridges which limit the groove are the metapleura of 

 the adult. Though at first solid, the connective tissue within 

 the ridge soon becomes hollowed and forms a lymph-space, 

 which we have not traced into connection with the coelom. 

 These ridges can be traced from about the middle of the larva's 

 body forward towards the pharyngeal region, where they 

 diverge considerably from one another (PI. XXX, figs. A, B, C). 



