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F, M. BALFOUR. 
terestincT, but it is by no means certain that any conclusions 
can be directly drawn from them as to the actual origin of the 
mesoblast in the triploblastic forms, till we know from what 
diploblastic forms the triploblastica originated. All that they 
show is that any part of the mesoblast may have originated 
from either of the primitive layers. 
Fig. 10 . — The Transverse Sections through Embryos of Hydrophilus piceus. 
(After Kowalevsky.) A. Section through an embryo at the point 
where the two germinal folds most approximate. B. Section through 
an embryo, in the anterior region wher^ the folds of the amnion have 
not united, gg. germinal groove; me. mesoblast; am. amnion; yk. yolk. 
For further light as to the origin of the mesoblast, it is 
necessary to turn to its actual development. 
The following summary illustrates the more important 
modes in which the mesoblast originates. 
1. It grows inwards from the lips of the blastopore as 
a pair of bands. In these cases it may originate (1) from 
cells which are clearly hypoblastic, (2) from cells which are 
clearly epiblastic, (3) from cells which cannot be regarded as 
belonging to either layer. 
Mollusca. — Gasteropoda, Cephalopoda, and Lamellibran- 
chiata. In Gasteropoda- and Lamellibranchiata it some- 
times originates from a pair of cells at the lips of the 
blastopore, though very probably some of the elements sub- 
sequently come from the epiblast ; and in Cephalopoda as a 
ring of cells round the edge of the blastoderm. 
Polyzoa Entoprocta . — It originates from a pair of cells 
at the lips of the blastopore. 
ChcBtopoda. — Euaxes. It arises as a ridge of cells at the 
lips of the blastopore (fig. 3). 
