266 
F. M. BALFOUR. 
outgrowths of the archenteron, the cavities of which become 
the body cavity. 
Brachiopoda . — The walls of a pair of outgrowths form the 
whole of the mesoblast. 
Chcetognatha . — The mesoblast arises in the same manner 
as in the Brachiopoda (fig. 12). 
Echinodermata . — The lining of the peritoneal cavity is 
developed from the walls of outgrowths of the archenteron, but 
the greater part of the mesoblast is derived from the amoeboid 
cells budded off from the walls of the archenteron (fig. 13). 
Enteropneusta {Balanoglossus ) , — The body cavity is de- 
rived from two pairs of alimentary diverticula, the walls of 
Avhich give rise to the greater part of the mesoblast. 
I'lG. 12. — Three Stages m the Development of Sagitta. (A and C after 
Butschli, and B after Kowalevsky.) The three embryos are represented 
in the same positions. A. Represents the gastrula stage. B. Repre- 
sents a succeeding stage, in which the primitive archenteron is com- 
mencing to be divided into three. C. Represents a later stage, in 
which tlie mouth involution {vi) has become continuous with alimentary 
tract and the blastopore has become closed, m. mouth ; al. alimentary 
canal ; ae. archenteron , bl.py blastopore ; pv. perivisceral cavity ; 
sp. splanchnic mesoblast ; so. somatic mesoblast ; ge. generative organs. 
Chordata. — Paired archenteric outgrowths give rise to the 
whole mesoblast in Amphioxus (fig. 14), and the mode of 
formation of the mesoblast in other Chordata is probably 
secondarily derived from this. 
3. The cells which will form the mesoblast become 
marked out very early, and cannot be regarded as definitely 
springing from either of the primary layers. 
Turhellaria. — Leptoplana (fig. 15), Planaria polychroa (?) 
Chcetopoda. — Lumbriciis, &c. 
iHscophora. 
It is very possible that the cases quoted under this head 
ought more properly to belong to group 1. 
