1890.] Development of the Atrial Chamber of Ampbioxus. 85 



The most serious error in Kowalewsky's view lies in the fact that 

 he makes the space in the lateral outgrowths continuous with the 

 body-cavity, and consequently calls it " Leibeshohle," or coelom. 



This space, as we shall see, does not belong to the true coelom, and 

 is not traceable as a space to the original myocoelomic pouches, but 

 ar ; ses apparently as an inter-cellular space in the midst of the con- 

 nective tissue in fact, it would seem to belong to that category of 

 spaces to which the term " pseudoccel " has been applied. In this 

 respect it stands in contrast to the spaces in connexion with the 

 dorsal and ventral fins, which have been shown by Hatschek to be 

 derived directly from the myoccelomic 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 ccelom 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 

 Kowalewsky'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 

 Kowalewsky that the epiplenra are comparable to the opercula of 

 Teleostean fish. 



I now propose to show that this view is based on erroneous 

 observation. 



Formation of the Atrial Chamber as observed by me. 



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 upstanding 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 never has any obvious connexion 

 with the ccalom. 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. That belonging to the animal's 

 left side keeps a more or less median position, and can be traced 

 (though but small in elevation) when twelve gill-slits are present as 

 a ridge situated at the lower or ventral margin of the gill-slits and 

 dying out in the anterior region of the pharynx. The right-hand 

 ridge, or metapleur, takes a course to the right of the gill-slits (which, 

 it will be retnemb'ered, are on the right side of the body), and over- 

 hanga the upper limit of the slits to a small extent. It dies out in 

 front of the first gill-slit, where it bends towards the middle line. 



