25 



endodenu. Whilst the secondaiy octodeini is making its 

 appearance the hinder portion of the archenteron becomes 

 shut off from the more anterior part by a coalescence of 

 some of its cells (PI. IV., fig. 9), resulting in the forma- 

 tion of a posterior intestinal portion with a closed cavitv, 

 and an anterior (esopliageal i)ortion whose lumen opens 

 to the exterior hx the blastopoie. The last-named 

 eventually becomes the mouth of the adult. Later the 

 cavities of the «soj)hagus and intestine become secondarily 

 continuous, but before this occurs a small lateral evagina- 

 tion is formed on either side of the inner portion of the 

 cesophagus. These evaginations eventually lose their 

 connection with the oesophagus, and acquire openings to 

 the exterior through the body wall. In this way are 

 established the nephridia. Thus the oesophagus must be 

 looked upon as endodermal, and consequently the 

 nephridia as diverticula of the archenteron. The anus is 

 formed later at the posterior end of the intestine. During 

 these changes the ectoderm of the proboscidial plate has 

 formed an invagination, which will become the lining of 

 the rhynchodieum and of the proboscis (PI. 1\'., hg. 10, /y.) 

 The embryo now presents the appearance shewn in PI. 

 IV"., fig. 5, and is known as Desor's larva. 



The fate of the ectoderm and of the endoderm has now 

 been traced. With the exception of the gonads, the 

 remainder of the body is derived from the mesoderm^ 

 whose cells by this time have come together to form a 

 continuous layer round the stru.ctures w^hose formation 

 has already been described. The mesoderm gives rise to 

 the connective tissues, muscles, nervous system, blood 

 vessels, and proboscis sheath. The cavities of the two 

 last are remains of the segmentation cavity. That the 

 nervous system should be of mesodermal origin is a some- 

 what remarkable fact. Still it has already been seen that 



