452 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



the interpretation of the process as observed in other vertebrates. The 

 most important point is that in Amphioxus during its early development 

 the mesoderm passes through a stage in which it consists of pocket-like 

 projections of the endoderm, arranged in a row along each side of the 

 body (Fig. 189, B). As development proceeds these enteroeoelic pockets 

 as they are tej-med become gradually nipped off from the endoderm so 



.^■tnes 



ent 



N 



Fig. i3g. 



Transverse sections of young Amphioxus illustrating the origin of tlie mesoderm. (After Hatsclielc. 

 ect, Ectoderm ; £»/, enteric cavity ; m.p, meduDary plate ; mes, mesoderm ; A^, notochordai 

 rudiment. The dark tone indicates ectoderm, the pale tone endoderm, and the medium tone 

 mesoderm. 



as to form closed compartments (Fig. 189, C). Each of these compart- 

 ments is a mesoderm segment, and its cavity is a coelomic cavity. 



By these phenomena in the development of Amphioxus we are taught 

 two important lessons in the morphology of vertebrates, namely (i) that 

 the coelome is enteroeoelic in origin, its mesoderm wall being derived 

 from part of the endoderm and (2) that the coelome is at an early stage 

 in its development divided into successive compartments as is the case 

 in an annelid worm. 



