DEVELOPMENT OF BALANOGLOSSUS KOWALEVSKII. 95 
tact with the ectoderm. These cells are in some parts only 
one layer thick (anterior and posterior walls) (fig. 8), while in 
others (ventrally) they proliferate rapidly (fig. 9), forming loose 
masses of cells, the outer elements of which are elongated, 
with rounded heads from which the spherical cells which form 
the inner portion are budded. This proliferation continues 
until the proboscis cavity is partially filled up. But in the 
later phases of Stage F elements are formed other than the 
rounded cells above mentioned, in the shape of fibres (fig. 13) 
which appear to arise in a curious way, as is shown in figs. 9, 
10, 12, &c. The elongated cells gradually become pyriform, 
the round ends being for the most part central, while the fine 
ends are drawn out into peripheral fibres. The round heads 
then appear to separate from the fibres, so that in examining 
the mesoblastic structure lining this cavity in late larve of 
Stage G, the elements are arranged in the following order : 
centrally, a small empty cavity surrounded by a ring of sphe- 
rical granular cells; next a layer of pear-shaped cells con- 
tinued into peripheral fibres, and externally a layer, composed 
almost, and later in life entirely, of radial fibres. Some of 
these are inserted into the lower layer of the skin, and are 
probably a peculiar form of connective tissue. In the heads of 
the pyriform cells brightly refractive granules may be seen; 
whether these are food or waste products cannot be affirmed. 
Such then is the lining of the proboscis-cavity. In the 
anterior third it is evenly distributed over the inner surface, 
but at the back of the posterior third, where the proboscis 
tapers abruptly to its stalk, the layer of mesoblastic tissue is 
much thinner dorsally than laterally and ventrally (compare 
fig. 13 which is through this region with figs. 12 and 11 which 
are anterior to it). On passing further backwards, the cavity 
in Stage Gis divided into two by a great proliferation of 
mesoblast from the dorsal surface, which grows downwards 
until it meets the ventral mesoblast. The position occupied 
by this structure at first coincides with the point at which the 
anterior mesoblastic pouch closed off from the archenteron. 
In that stage the anterior body cavity was a simple sac con- 
