270 
F. M. BALFOUU. 
the process observable in Amphioxus ; but this process 
approaches in many forms of Vertebrata to the ingrowth of 
the mesoblast from the lips of the blastopore. 
It is, therefore, highly probable that the paired ingrowths 
of the mesoblast from the lips of the blastopore may have 
been in the first instance derived from a pair of archenteric 
diverticula. This process of formation of the mesoblast is, 
as may be seen by reference to the summary, the most 
frequent.^ 
While there is no difficulty in the view that the body 
cavity may have originated from a pair of enteric diverticula 
in the case of the forms where a body cavity is present, there 
is a considerable difficulty in holding this view, for forms 
in which there is no body cavity distinct from the alimen- 
tary diverticula. 
Tig. 17 . — Section through the Embryo of Agelena labyrinthica. The section 
is represented with the ventral plate directed upwards. In the ventral 
plate is seen a keel-like thickening, which gives rise to the main mass of 
hie mesoblast. yk. Yolk divided into large polygonal cells, in several of 
which are nuclei. 
Of these types the Platyelminthes are the most striking. 
It is, no doubt, possible that a body cavity may have existed 
^ The wide occurrence of this process was first pointed out by E.abl. He 
holds, however, a peculiar modification of the gastraea theory, for which I 
must refer the reader to his paper (No. 23) ; according to this theory the 
mesoblast has sprung from a zone of cells of the blastosphere, at the 
junction between the cells which will be invaginated and the epiblast 
cells. In the bilateral blastosphere, from which he holds that all the 
higher forms (bilateralia) have originated, these cells had a bilateral 
arrangement, and thus the bilateral origin of the mesoblast is explained. 
The origin of the mesoblast from the lips of the blastopore is explained by 
the position of its mother-cells in the blastosphere. It need scarcely be 
said that the views already put forward as to the probable mode of origin 
of the mesoblast, founded on the analogy of the Coelenterata, are quite 
incompatible with liabl’s theories. 
