22 
MONICA TAYLOR 
branchial rudiment is still solid as explained above. These 
ingrowths of ectoderm are quite masked externally by the 
outer horny layer, which is not involved in the ingrowth. If 
this deep-seated layer of ectoderm is a true ectoderm — and 
there seems no reason to doubt it — then it seems pretty clear 
that each of the branchial clefts has a lining of ectoderm. 
The pharyngeal rudiment behind the fourth branchial pouch 
tapers off to a solid cord of cells, the diameter of which 
gradually diminishes. Since the pericardial rudiment only 
extends to the commencement of the oesophagus, the ali- 
mentary canal beyond this point, while having no organic 
continuity with, yet lies directly over, the great yolk-mass. 
The differentiating stomach, presumably from lack of space in 
a ventral direction, expands laterally to the left. The dorsal 
mesenteries are for the same reason not vertical and median, 
but more or less horizontal. The liver appears at this stage 
as a solid lateral (to the left) outgrowth of the alimentary canal. 
On the right, dorso-lateral in position, is the dorsal pancreatic 
rudiment, solid, surrounded by and passing without any sharp 
boundary into the surrounding mesenchyme, which forms the 
spleen rudiment. This latter arises in connection with venous 
blood, which is part of the liver and yolk-sac circulation. It 
will be seen in the section on the vascular system that the 
subintestinal vein breaks up into capillaries on the yolk-sac, 
and that as regards circulation the liver forms part of the yolk. 
The spleen, then, has the usual teleostean connection with the 
blood of the subintestinal vein. 
In Stage 25 the operculum — a typical teleostean one — has 
grown backwards to the level of the second gill-arch, and the 
gill-cavity begins to differentiate. Chinks appear in the solid 
cleft rudiments separating apart the flat epithelium lining of 
the two sides of each slit. The arches are not at this stage 
the long curved structures which they eventually become ; 
almost the whole of one arch is visible in one transverse 
section. The branchial rudiment no longer projects to the 
exterior as it did in earlier stages. A study of PI. 3, fig. 20, a 
horizontal section through an embryo of Stage 25, will show 
