THE MKSODBRjr. 133 



a secretion of the endoderm cells. The contents of the ali- 

 mentary canals of the free-living adults is permeated by a 

 number of similar highly refracting bodies. 



The Mesoderm. 



The later history of the mesoderm, i. e. the tissues derived 

 from the walls of the somites, will best be considered under 

 the following four heads, viz : 



1. The muscles. 



2. The vascular system. 



3. The nephridia. 



4. The generative organs. 



Whether any part of the cutaneous mesodermal structures 

 are ectodermal in origin is, as I have already hinted in dealing 

 with the nervous system, impossible to decide, because of 

 the intimate connection which is established between the ecto- 

 derm and the somatic walls of the somites, at almost the first 

 appearance of the latter (vide pp. 60, 70), and which remains 

 during the whole development (vide p. 131). 



The Muscles. 



The cutaneous muscles arise from the subectodermal fibrous 

 network which has been already mentioned, and which in Stage r 

 was crowded with the nuclei of the ventro-lateral corners of 

 the somites. The fibres of the outer part of this network 

 arrange themselves in a circular manner, and form the circular 

 muscles of the body wall. At first the fibres are extremely 

 scanty (PL X, fig. 5, circ. muse), but they soon become 

 more numerous. Nuclei are found at intervals amongst them 

 (PI. XII, fig. 13). The longitudinal muscles appear some- 

 what later within the circular muscle in seven patches, viz. two 

 dorsal (PI. XII, fig. 13), two lateral, two ventro-lateral, and 

 one medio-ventral between the nerve-cords. These patclies 

 gradually enlarge into the corresponding muscular bands of the 

 adult. They contain nuclei which have often a peculiar, irre- 



