4 
ME. E. EAT LANKESTEE ON THE 
seen in figs. 9, 10, 11. There can, I think, be little doubt, after an examination of these 
figures and fig. 11, that some of these mesoblastic cells are proliferated from the epiblastic 
wall. In particular I may draw attention to fig. 13, where the continuity of some of 
the branching corpuscles of the mesoblast with the granular matter surrounding the 
large nuclei of the epiblast, two of which are seen (ep) in the figure, is obvious. It is 
perhaps necessary again to mention that these different views (figs. 10-13) are taken 
from the same embryo (without shifting its position) by altering the focus, a power of 
1100 diameters being employed so as to obtain a series of optical sections. 
It is a more doubtful matter as to whether any of the mesoblastic cells are derived 
from the invaginated block of hypoblastic corpuscles. In figs. 11 & 13 I would draw 
attention to the corpuscles marked p, which appear to be in the act of detaching 
themselves from the hypoblast, whilst the corpuscle (pd) has the appearance of a hypo- 
blastic cell undergoing quadruple division. 
It is not desirable here to summarize or discuss the various views now current as to 
the origin of the mesoblast. It is sufficient to say that the derivation of a portion of 
the mesoblast from the epiblast, and of another portion from the hypoblast is in accord- 
ance with the view most recently adopted, from various considerations, by Professor 
Eknst Haeckel*. At the same time we are by no means yet in a position to assert 
that the mesoblast has uniformly the same origin in the various classes of the animal 
kingdom, nor in all members of the same class, though this uniformity should be our 
working hypothesis. 
Plate 2. fig. 18 represents in optical section an embryo somewhat more advanced 
than that of figs. 9-11, and with a consequently larger development of mesoblastic cor- 
puscles in the cavity lying between epiblast and hypoblast.' 
Plate 2. fig. 19 exhibits this in optical section taken just below the epiblastic 
surface. 
Plate 2. fig. 20 advances to a later stage. The hypoblast is now seen to be assuming 
a definite form. Seen thus in optical section, it appears as a bilobed mass supported by 
a peduncle, rp. This peduncle f develops subsequently into the rectum, and may there- 
fore be designated the “ rectal peduncle.” 
The cells or nucleated corpuscles of the mesoblast have greatly augmented in number, 
and in this particular view those especially are obvious which, accumulating at the 
pole opposite to the attachment of the rectal peduncle, lay the foundation of the foot ( f ). 
Plate 2. fig. 21 shows epiblast and hypoblast in optical section, and is introduced 
to demonstrate the mobility of the walls of the vesicular embryo. Active movement 
does not occur; but slow changes of long and short diameter are noticeable at this 
period of development. 
Plate 2. figs. 22 & 23 represent an embryo in two slightly differing depths of optical 
* Die Gastrsea-Theorie, die phylogenetische Classification des Thierreichs, und die Homologie der Keimblatter. 
Jena, September 1873. 
t I bave elsewhere applied the term “ pedicle of invagination ” to this same group of cells. 
