26 h 
F. M. BALFOUR. 
show that there are a greater number of instances in which 
the mesoblast has a mixed origin than might be supposed 
from the above summary. 
Fig. 14. — Sections of an Amphioxus 'Embryo at three Stages. (After Kow- 
alevsky.) A. Section at gastrula stage. B. Section of a somewhat 
older embryo. C. Section through the anterior part of still older 
embryo, np. neural plate ; n.c. neural canal ; rues, archenteron in A, 
and mesenteron in B and C ; ch. notochord ; so. mesoblastic somite. 
I have attempted to reduce the types of development of 
the mesoblast to six ; but owing to the nature of the case it 
is not always easy to distinguish the first of these from the 
last four. Of the six types the second will on most hands 
be admitted to be the most remarkable. The formation of 
hollow outgrowths of the archenteron, the cavities of which 
give rise to the body cavity, can only be explained on the 
supposition that the body cavity of the types in which 
such outgrowths occur are derived from diverticula cut off 
from the alimentary tract. The Iming epithelium of the 
diverticula — the peritoneal epithelium — is clearly part of the 
primitive hypoblast, and this part of the mesoblast is clearly 
hypoblastic in origin. 
Fig. 15. — Sections through the Ovum of Leptoplana tremellaris in three 
Stages of Development, (After Hallez.) ep. epiblast ; m. mesoblast ; 
hy. yolk cells (hypoblast) ; bl. blastopore. 
In the case of the Chsetognatha (Sagitta), Brachiopoda, 
and Amphioxus, the whole of the mesoblast originates from 
the walls of the diverticula ; while in the Echinodermata the 
walls of the diverticula only give rise to the vaso-peritoneal 
e])itheliuin, the remainder of the mesoblast being derived from 
amoeboid cells which spring from the walls of the archenteron 
