300 POLYGORDIUS 



adjacent ones before and behind : in this way the septa 

 arise. 



The nephridia appear to have a double origin, the super- 

 ficial portion of each being formed from ectoderm, the 

 deep portion including the nephrostome from the somatic 

 layer of mesoderm. 



In the ventral nerve cord the cells lying nearest the outer 

 surface have enlarged and formed nerve-cells, while those on 

 the dorsal aspect of the cord have elongated longitudinally 

 and become converted into nerve fibres. This process has 

 already begun in the preceding stage. 



But the most striking histological changes are those which 

 gradually take place in the somatic layer of mesoderm. At 

 first this layer consists of ordinary nucleated cells (o 1 , Msd 

 (soni) ), but before long each cell splits up in a radial 

 direction (D 2 ) from without inwards i.e. t from the ectoderm 

 (Der. Epthm} towards the coelome finally taking on the 

 form of a book with four or more slightly separated leaves 

 directed outwards or towards the surface of the body, and 

 with its back the undivided portion of the cell bounding 

 the coelome. The cells being arranged in longitudinal series 

 we have a number of such books placed end to end in 

 a row with the corresponding leaves in contact page one 

 of the first book being followed by page one of the second, 

 third, fourth, c., page two by page two, and so on through 

 one or more segments of the trunk. Next what we have 

 compared with the leaves of the books the divided 

 portions of the cells become separated from the backs- 

 the undivided portions (D S ) and each leaf (M. PI) fuses 

 with the corresponding leaves of a certain number of books 

 in the same longitudinal series. The final result is that the 

 undivided portions of the cells (backs of the books, CceL 

 Epthni) become the parietal layer of coelomic epithelium, the 



