262 
F. M. BALFOUR. 
Tlie organs invariably derived, in the triploblastic forms, 
from the mesoblast, are the vascular and lymphatic systems, 
the muscular system, and the greater part of the connective 
tissue and the excretory system. On the other hand, the 
nervous system and organs of sense (with a few possible 
exceptions), the epithelium of most glands, and a few ex- 
ceptional connective-tissue organs, i. e. the notochord, are 
developed from the two primary layers. 
The fact of the first-named set of organs being invariably 
derived from the mesoblast.points to the establishment of the 
two following propositions : — (1) That with the differentiation 
of the mesohlast as a distinct layer hy the process already 
explained, the two primary layers lost for the most part the 
capacity they primitively possessed of giving rise to muscular 
and connective-tissue differentiations. (2) That the mesohlast 
throughout the triploblastic Metazoa, in so far as these forms 
have sprung from a common triploblastic ancestor, is an 
homologous structure. 
The second proposition follows from the first. The meso- 
blast can only have ceased to be homologous throughout the 
triploblastica by additions from the two primary layers, and 
the existence of such additions is negatived by the first pro- 
position. 
These two propositions, which hang together, are possibly 
only approximately true. In the first place, it is quite 
possible that fresh differentiations from the two primary 
layers may have arisen after the triploblastic condition had 
been established, and by the process of simplification of 
development and precocious segregation, as Lankester calls 
it, have become merged in the normal mesoblast ; or such 
differentiations may have taken place in forms, the deve- 
lopment of which has not yet been investigated. Had this, 
however, been a frequent occurrence, it is hardly likely that 
no instance of it should have been recorded for the muscular 
system and connective tissue so that it is probable that 
the muscular system of all existing triploblastic forms has 
been differentiated from the muscular system of the an- 
cestor of the triploblastica. In the case of other tissues 
there are a few instances Avhich might be regarded as 
examples of an organ primitively developed in one of the 
two primary layers having become secondarily carried 
into the mesoblast. The notochord has sometimes been 
cited as such an organ, hut it now appears probable that its 
hypoblastic origin can always be demonstrated. The de- 
^ The connective-tissue test of the Tunicata, though derived from tlie 
cjpiblast, is not really an example of such a differentiation. 
