DEVELOPMENT OF BALANOGLOSSUS KOWAILEVSKII. 93 
traction which the body may undergo in preservation. The 
trouble chiefly occurs in the case of the proboscis-stalk and 
folds of the collar. By comparing figs. 3 and 4, which were 
drawn from living specimens in the extended state, with fig. 5, 
which is taken from a preserved specimen, these relations will 
be understood. In the contracted animal the gill-slits are 
always more or less obscure, owing to the great shortening 
which takes place in the branchial region, together with the 
protrusion of the valves (fig. 5, viv.). 
In older animals this contraction is not nearly so great, 
probably owing to the increased firmness of the branchial 
skeleton. 
Internal Structure. 
Stages Fand G—Skin.—The ectoderm is composed of long 
fusiform cells arranged two to three deep over the body, except 
in the dorsal side of the collar groove, where they are colum- 
nar and one layer thick. Also in the posterior dorsal region 
the skin is thinner than that of the rest of the body. The 
whole surface is ciliated. Beyond the fact that the cells are 
more compressed, and closely arranged, the structure is 
similar to that of the previous stage. 
Nervous System.—The solid cord which began to separate 
from the skin in the middle dorsal line at Stage F continues to 
sink inwards. No lumen is as yet present in it. Throughout 
its length it still remains in contact with the skin, fusing with 
it at both ends (figs. 10 and 20—24). 
Towards the end of Stage G a differentiation begins between 
the upper and lower parts of this cord, the upper being formed 
of cells, while the lower part consists of lightly stained sub- 
stance, which in older animals is distinctly made up of fibres. 
Its fibrous nature cannot in this condition be certainly affirmed, 
probably owing to defect in preservation. The formation of a 
nervous network over the whole body, which afterwards 
occurs, is not yet begun. 
Hy poblast.—The mouth is ventrally directed (fig. 7), and 
