THE DEVELOPMENT OF SYMBRANCHUS MARMORATHS. 29 
lateral, both anterior and posterior parts lie between the 
alimentary canal and the liver (PI. 4 , fig. 24). 
The spleen is a compact, lenticular-shaped structure more 
posterior than the gall-bladder, lying near the liver to tlie 
right of the intestine. 
Yolk and Alimentary Canal. 
In Sym branch us, as elsewhere, in the early stages, 
centres of growth are characterised by the presence of yolk, 
which is being reduced to fine dust-like particles by the 
activity of the intermingled protoplasm. In Stages 18, 21 and 
22 large quantities of such yolk in process of assimilation 
with large, deeply staining yolk-nuclei ai-e to be found under 
the hind end of the embryo. As the embryo increases in size 
a strand of this yolk-syncytium, with its characteristic nuclei, 
extends into the free tail part of the embryo immediately 
over the differentiating subintestinal vein. PL 4, fig. 25 is a 
sagittal section through an embryo of Stage 24. A somewhat 
similar condition of things is described by Assheton (2) for 
Gymnarchus niloticus. Similarly in later stages an 
anterior plug is visible (Stages 29, 30 and 31) behind the peri- 
cardium. This plug (PI. 4, fig. 26) in Stage 31 becomes more 
or less separated from the rest of the yolk, and has much the 
appearance of a group of round and highly reticulate large 
nucleated egg-cells. These cells lie in the mesenchyme 
immediately posterior to the pericardium and ventral to the 
hinder part of the oesophagus. 
Pharyngeal Pouch. 
The whole of the lining of the gill-chamber of Sym- 
branchus as well as the roof and floor of the mouth is 
provided with a rich network of blood-vessels forming a 
respiratory tissue. Anterior to the first efferent branchial the 
aortic roots of each side give off a large artery which breaks 
up into innumerable capillaries in the roof of the mouth. This 
blood is drained back into the anterior cardinals. 
