368 
MESSRS. F. M. BALFOUR AND W. N. PARKER ON THE 
with, in the early stages of these structures by Marshall in Birds,'"' and by one of us 
in Elasmobranchs.t 
In the parietal region a cavity has now appeared in part of the trunk between the 
splanchnic and somatic layers of the mesoblast (Plate 23, fig. 29, b.c.), the somatic layer 
(so.) consisting of a single row of columnar cells on the dorsal side, while the remainder 
of each somite is formed of the splanchnic layer (sp.). In many of the sections the 
somatic layer is separated by a considerable interval from the epiblast. 
We have been able to some extent to follow the development of the segmental duct. 
The imperfect preservation of our specimens has, as in other instances, rendered the 
study of the point somewhat difficult, but we believe that the figure representing the 
development of the duct some way behind its front end (Plate 23, fig. 29) is an accurate 
representation of what may be seen in a good many of our sections. 
It appears from these sections that the duct (Plate 23, fig. 29, sg.) is developed as a 
hollow ridge-like outgrowth of the somatic layer of mesoblast, directed towards the 
epiblast, in which it causes a slight bulging. The cavity of the ridge freely communi¬ 
cates with the body-cavity. The anterior part of this ridge appears to be formed first. 
Very soon, in fact, in an older embryo belonging to this stage, the greater part of the 
groove becomes segmented off as a duct lying between the epiblast and somatic meso¬ 
blast (Plate 23, fig. 28, sg.), while the front end still remains, as we believe, in com¬ 
munication with the body cavity by an anterior pore. 
This mode of development corresponds in every particular with that observed in 
Teleostei by PvOSENBErg and Oellacher. 
The structure of the notochord (nc.) at this stage is very similar to that observed 
by one of us in Elasmobranchii.t The cord is formed of transversely arranged flat¬ 
tened cells, the outer parts of which are vacuolated, while the inner parts are granular, 
and contain the nuclei. This structure gives rise to the appearance in transverse 
sections of an axial darker area and a peripheral lighter portion. 
The hypoblast retains for the most part its earlier constitution, but underneath the 
notochord, in the trunk, it is somewhat thickened, and the cells at the two sides spread 
in to some extent under the thickened portion (Plate 23, fig. 29, s.nc.). This thicken¬ 
ing, as is shown in transverse sections at the stage when the segmental duct becomes 
separated from the somatic mesoblast (Plate 23, fig. 28, s.nc.), is the commencement of 
the subnotochorclal rod. 
The tail end of the embryo still retains its earlier characters. 
Seventh day after impregnation .—Our series of specimens of this stage is very 
imperfect, and we are only able to call attention to the development of a certain 
number of organs. 
Our sections clearly establish the fact that the optic vesicles are now hollow processes 
* Journal of Anat. and Physiol., vol. xi., p. 491, plates xx. and xxi. 
t ‘Elasmobranch Fishes,’ p. 156, plates x. and xiii. 
% Ibid., p. 136, plate xi., fig. 10. 
