248 
F. M. BALFOUR. 
would venture to assert that any hypotheses, which can now 
be put forward as to the mode of origin or the homologies of 
the germinal layers, have more than a tentative value. 
In the following pages I aim more at summarising the 
facts, and critically examining the different theories which 
can be held, than at dogmatically supporting any definite 
views of my own. 
In all the Metazoa the development of which has been in- 
vestigated the first process of differentiation which follows 
upon the segmentation consists in the cells of the organism be- 
coming divided into two groups or layers, knowm respectively 
as epiblast and hypoblast.'' This process may commence 
during later phases of the segmentation, but is generally not 
completed till after the close of the segmentation. Not 
only do the cells of the blastoderm become differentiated 
into two layers, but these two layers, in the case of a very 
large number of ova wdth but little food-yolk, constitute a 
double-w^alled sack — the gastrula (fig 1) — the characters of 
Fig. 1. — Diagram of a Gastrula. (From Gegenbaur.) a. Moutli ; h. 
archenteron ; c. hypoblast ; d. epiblast. 
which are too well known to require further description. 
It is generally admitted, and will be assumed in the sequel, 
that the segmented ovum represents phylogenetically a 
compound Protozoon ; and following the same lines of 
phylogenetic speculation, it may be concluded that the two- 
layered condition of the organism represents in a general 
way the passage from the Protozoon to the Metazoon 
condition. It is probable that we may safely go further, 
and assert that the gastrula reproduces, wdth more or less 
fidelity, a stage in the evolution of the Metazoa, permanent 
in the simpler Hydrozoa, during which the organism w^as 
^ 111 a few cases amongst the Mollusca and Chaetopoda, &c., the meso- 
blast is differentiated simultaneously with the two other layers. These 
cases may for the moment be left out of consideration. 
