340 s. HATTA : 



selerotomic layer forms, as has been seen above, a direct continua- 

 tion of the visceral layer giving rise to the coelomic projection. 

 (The successive changes of the myotome are seen in figs. 1, 2, 3, 

 18, 21, 22, 43, 49, 60, &c.) 



From the above account, it can be inferred that the ventral half 

 of the mesoblastic somite in Petroinyzon, which gives rise to the 

 pronephic Anlage and the coelomic projection, is doubtless homo- 

 logous with the " inicrmcdiaie cell-mass " of Balfouk described by 

 liim in Selachia and, therefore exactly coincides with the ** Nephro- 

 tom " of Ki'CKERT.^* Ho far as concerns its future destination, how- 

 ever, the results arrived at by me slightly deviate from their views. 



A series of sections through the oldest embryo of this stage 

 is represented in figs. 64-76, The epiblast has undergone no 

 histological change, but remains, as before, one cell deep. Many 

 structures, however, exhibit a remarkable progress. The muscle- 

 layer {iiiua.) of the myotome is, for instance, further difierentiated, 

 now consisting of a transverse row of long nniscle-cells, although 

 the cutis-layer {rut.) is still composed of short cubical cells. In the 

 anterior region, the true coelome {pp-c), l)ecomes conspicuous en- 

 closed l)y the parietal {m./>.) and the visceral {i/).r.) layers of the 

 lateral [)late, both of which consist of a single row of cubical cells. 

 The ventral edges of the lateral plates on both sides do not, however, 

 yet meet in the ventral median line. The walls of the enteric 

 canal too are, in the anterior region, reduced into a single cell layer. 



A great alteration is met with in the prouephric tubules. 

 They have assumed a cylindrical form composed of tall columnar 

 epithelium and have grown dorsally, pushing in between the 

 myotome and the epiblast, causing the latter to be elevated a 



1) In si)ite of the discussion liy Kl'CKEkt ('89, jip. l'J-20) on the inexactness of the expres- 

 sion "inlerniediule cell-muss," I honiologise, witli many aulhois, ihese l\\o tcnns with each othei'. 



