DEVELOPMENT OF BALANOGLOSSUS KOWALEVSKII. 97 
mesoblast cells which are in contact with the hypoblast into 
curious tails, which are refractive. 
I was at first led to suppose that these tails were mus- 
cular, and this undoubtedly is the fate of many of the 
mesoblastic elements of this region, but no direct evidence of 
the contractile nature of these pear-shaped cells was attained 
beyond the general suggestion of their shape; on the other 
hand, when the notochordal sheath is developed in this region, 
an appearance is presented which suggests the possibility of 
their having taken part in its formation. Their histological 
characters are, however, strikingly similar to those of the cells 
which occur at the sides of the notochord in the same region 
in Amphioxus at the time when it has eleven pairs of meso- 
blastic pouches (Hatschek, No. 4, figs. 124, 126, &c.). These 
cells are stated by Hatschek to be muscle-fibres; by analogy 
it seems, therefore, likely that this may be the real nature of 
the same cells in Balanoglossus. 
As development proceeds, proliferations of mesoblast are 
formed in the ventral region of the middle body cavities. From 
each side in the posterior region of these cavities a tubular 
portion is separated off from the rest (fig. 28). 
The fate of these parts is not quite certain. It seems, how- 
ever, likely that they unite with two forward growths from the 
posterior body cavities to form the tissue space in which the 
dorsal blood-vessel is ultimately enclosed throughout the collar 
region. This tissue space I propose to call the perihema 
cavity (fig. 60, &c., Ph. c.). 
The posterior body cavities are simple cavities, similar 
to the middle pair. The dorsal and ventral mesenteries 
persist throughout life. As yet they contain no special 
differentiations. 
Period between the Formation of the First and 
Second Pair of Gill-Slits. 
Skin and Nervous System.—The histology of the skin 
in the proboscis and collar regions has not undergone material 
7 
