THE ENTBEOPNEUSTA. 23 



(1) The peripheral part consists of cells and fibres. The cells 

 are mostly pyriform similar to those of one-gill larvae. 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 as a 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 

 larvae, 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. 47 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 diflferentiated. 



