XXV METAMORPHOSIS 301 



thus quite a different thing from the larval body-cavity 

 or blastoccfile, being formed, not as a space between 

 ectoderm and endoderm, but by the splitting of an 

 originally solid mesoderm. 



The division of the mesoderm does not however extend 

 quite to the middle dorsal and middle ventral lines : in both 

 these situations a layer of undivided mesoderm is left (c), 

 and in this way the dorsal and ventral mesenteries are 

 formed. Spaces in these, apparently the remains of the 

 blastocoele, form the dorsal and ventral blood-vessels. More- 

 over the splitting process takes place independently in each 

 segment, and a transverse vertical layer of undivided 

 mesoderm (b, Sep) is left separating each segment from the 

 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 (d^, Msd 

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

 direction (d^) from without inwards — i.e., 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 



