MORPHOLOGY OF CYCLOSTOMATA. 339 



;i true fold of the visceral layer of the lateral plate. At the 

 same time, the upper angle or the pronephric angle develops 

 further and assumes a tubular form composed of a single layer 

 of columnar cells {G). 



The peritoneal cavity begins, therefore, at the point, from 

 which the coelomic projection starts, and the part of the layer 

 dorsal to this point is all appropriated to the formation of the 

 pronephric tubule. The nephrostome will be found, therefore, by 

 the point where the tubule passes over to the projection. 



I will add a few words on the diftereutiation of the myo- 

 tome, so far as concerns the topographical relation of it to the 

 Anlage of the pronephros. The myotome consists, at the present 

 stage (Stage ui), of the inner and outer layers which constitute 

 respectively the Mu.^helblatt and the Cutuhlatt of German 

 authors (lig. 59 and 60, nms. and cut.). The cells composing the 

 Mu^helblait {mua^ are, simply differentiated into a transverse row 

 of the muscle-plates. The outer layer [cut^ undergoes, however, 

 subsequently a series of interesting changes : it folds in, just as 

 the Sklcrablatl or sclerotome described by Hatciiek in Amphioxua 

 ('88) between the Blushclblatt and the chorda and the neural tube.'' 

 As is well known, Rabl ('88) has homologised Hatcher's Sklerab- 

 latl with his Sdcroionidivcrtikcl of Selachian embryos, which is 

 the evagination of the ventral part of the visceral layer of the meso- 

 blastic somite. This part of the somite (the sclerotome) corres- 

 ponds, I believe, exactly to the ventral row of the pentagonal 

 myotome in my embryo (see pp. 314 and 320), which comes after- 

 wards to form the ventral part of the cutis-layer (see figs. 21, 22, 

 23, 36, 37, 43, 49, ö9, 60, &c.). When the myotome is not yet 

 separated from the rest of the mesoblast (fig. 2), this part of the 



1) This biihject will he trealeJ fif in an independent article. 



