DEVELOPMENT OF BALANOGLOSSUS KOWALEVSKII. 103 
(1) The peripheral part consists of cells and fibres. The cells 
are mostly pyriform similar to those of one-gill larve. They 
contain few granules and are much reduced in number. The 
fibres are more numerous. They are of several kinds: thick 
strands that are obviously muscular, running as yet only in a 
longitudinal direction, having their ends inserted in the skin: 
and also thinner fibres, which are arranged circularly, as in 
the periphery of the middle third; longitudinally, only 
occurring asa sort of sheath separating the circular fibres from 
the general connective tissue, and occasionally in the central 
portions; radially, as over the whole periphery of the pro- 
boscis cavity, and finally, very fine fibres forming a loose 
network connecting all the mesoblastic elements together. As 
may be seen in figs. 31, &c., there is as yet no suggestion of 
the peculiar concentric arrangement of the longitudinal fibres, 
which gives a section of the adult proboscis its curious appear- 
ance. The radial fibres are mostly inserted into the skin 
(fig. 32), their possible connection with the epiblast has been 
already discussed. 
There are still present a few of the large spherical cells 
which formed the inner layer of mesoblast in one-gill-slit 
larvee, but they contain fewer granules, and are only found in 
the posterior parts of the cavity. 
(2) The central portion in the one-gill-slit larva was made 
up of a mass of spherical and polygonal cells surrounding a 
central part filled with loose tissue (fig. 16). As the noto- 
chord grows forward, this structure, which, from its subsequent 
fate may now be termed the proboscis gland, is prolonged with 
it. As this forward growth occurs, a basement membrane 
forms between the mesoblast outside the proboscis gland and 
the loose tissue contained in it. Fig. 46 shows a section ante- 
rior to the part where this basement membrane is formed. 
The mesoblastic cells are here large and granular, having 
irregular shapes. The proboscis gland now is hollowed out, 
so that it contains a space which extends from the proboscis 
stalk to the front end of the notochord. This space contains a few 
mesoblastic cells, which are as yet not obviously differentiated. 
