604 E. W. MACBRIDE. 



of development is due to the relations of growth and pressure 

 subsisting between them. No one who has seen well-preserved 

 sections can doubt that it is by a folding process that the 

 mesoderm is formed in Amphioxus. 



Later the cavities of the somites enlarge, and the dorsal 

 portions of their inner walls are like the corresponding parts 

 of the collar cavities converted into longish muscles {Muse, 

 fig. 20, a, 6, and c), and form all the myotomes except the 

 first. The ventral portions wedge themselves in between the 

 gut and the posterior ventral extension of the collar cavities 

 (F. Tr., figs. 19, c, 20, a, b, and c). Whether in the living 

 condition these ventral portions are completely hollow, or 

 whether, as is suggested by the examination of sections, 

 they are partly represented by solid tongues of tissue, it is 

 impossible to settle. Of course in the adult the ventral por- 

 tions of the somites give rise to the dorsal coelomic canals, 

 and to the canals running in the primary and secondary gill 

 bars (Lankester) ; the dorsal coelomic canals are clearly repre- 

 sented in the larva, but I have found it impossible to be 

 certain whether the rest of the ventral portions of the somites 

 are open spaces in the larva or not. Just at the close of 

 embryonic life the '^ myotome " becomes separated from the 

 rest of the somite by a septum, and the ventral portions of the 

 somites acquire communication with each other, about the 

 region where the dorsal coelomic canals afterwards appear. 

 This ventral fusion of the somites was inferred by Hatschek 

 from the fact that he could not trace the dividing lines 

 between the somites to the mid-ventral line. The specimens 

 of later larvae which were well enough preserved to rely on 

 showed the trunk coelom (derived from the ventral fusion of 

 the somites) clearly only behind the gill-slits, where the 

 collar cavities were dying out. Elsewhere the extreme 

 difiiculty of staining the connective tissue and peritoneal epi- 

 thelium made it impossible to be certain whether a narrow 

 slit-like cavity or only a wedge of tissue intervened between 

 the gut wall and the collar cavity. 



