20 
MONICA TAYLOE. 
The opercular cavity stretches backwards to a distance of 
4 Him., the opercular opening being now quite the small 
median ventral aperture so typical of the adult. 
The ventral aorta is very long, the distance between the 
heart and the fourth aortic arch being 1*6 mm. In the full- 
grown specimen the distance between the tip of the snout and 
the pericardium is roughly a quarter of the whole length, 
and as a rule the pericardium bulges out slightly and is 
consequently visible exteriorly. The eyes are smaller in 
proportion than they were in the earlier stages. 
III. Notes on the Development of Certain Systems 
OF Organs. 
A. Alimentary Canal and Certain of its Glands. 
As has been shown in the preceding account, gastrulation 
occurs at about Stage 8. Celloidin sections through the egg 
of Sy mb ranch us at Stage 10 and onwards show that yolk 
endoderm (yolk syncytium) underlies the whole of the blasto- 
derm. In the embryonic rudiment (Stage 10) endoderm 
cells can be clearly distinguished from the overlying thick- 
ened neural plate (PI. 3, fig lS,n,j).). The nuclei of this endo- 
derm, which does not extend beyond the limits of the 
rudiment, in contradistinction to the large nuclei of the yolk 
endoderm, are small and oval-shaped, the long axis of each 
being usually at right angles to the axis of the embry- 
onic rudiment. The line of demarcation between the endo- 
derm of the embryonic rudiment and the yolk-syncytium is 
well defined in both parafiin and celloidin sections. 
As the development proceeds, mesoderm gradually separates 
apart the endoderm and the overlying neural rudiment, and 
the notochord is formed. I have not been able to observe 
the process by which the notochord is derived from the endo- 
derm, but by Stage 16 this structure is easily recognisable 
as a solid cord of cells. 
In Stage 18 the stomodmum begins to grow in from the 
deep layer of ectoderm. It is very difficult to fix precisely 
