ORIGIN OF THE MESODERM 



57 



mes. 



enl. 



D 



N 



Upon the former hypothesis the extension of the mesoderm later- 

 ally by delamination from the endoderm, which does certainly occur 

 in some forms (see below), would be an inexplicable mystery. On the 

 second hypothesis on the other hand this splitting off of mesoderm 

 would be comparable with a gradual deepening of the angle which 

 bounds the mesodermal pocket of Amphioxus on its mesial side (see 

 Fig. 34, B). The dorsal attachment is, on this view, to be regarded 

 as a secondary fusion between mesoderm and endoderm. In the 

 higher vertebrates 



this region becomes *, fn.p. ec f- 



the seat of active 

 cell proliferation 

 which plays a great 

 part in the produc- 

 tion of mesoderm. 



After these 

 general remarks we 

 may proceed to con- 

 sider shortly the 

 details of the early 

 development of the 

 mesoderm in a few 

 examples of the 

 lower Vertebrata. 



Amphioxus. — 

 In Amphioxus the 

 development of 

 mesoderm begins 

 with the formation 

 of a longitudinal 

 fold or outpushing 

 of the endoderm on 

 each side of the 

 mid -dorsal line 

 (Fig. 34, A, mes). 

 In this way there 

 is formed on each 

 side an upwardly - 



projecting groove or gutter, the narrow cavity of which is a pro- 

 longation of the archenteric cavity (Fig. 34, B). Constrictions 

 appear now in the wall of this gutter which divide it up into 

 successive segments — the constrictions developing in order from 

 the head-end backwards. The groove or fold is in this way con- 

 verted into a series of pockets the coelomic or enterocoelic pouches. 

 The cavity of these pouches except in the case of the first two 

 usually becomes for a time practically obliterated by the outer 

 and inner walls coming into contact. Finally the communication 

 between pouch and archenteron becomes closed and the pouch 



Fig. 34. — Transverse sections of young Amphioxus illustrating 

 the origin of the mesoderm. (After Hatschek, 1881.) 



ect, ectoderm ; ent, enteric cavity ; m.p, medullary plate ; mes t 

 mesoderm ; N t notochordal rudiment. 



