486 ADAM SEDGWICK. 



mites (PI. XXXVI, fig. 32, and PI. XXXIV, figs. 14, 16 a, 

 &c.). Its further development will be followed in Part IV, 

 but I may now state that it becomes the pharynx and 

 oesophagus of the adult. 



Behind, the walls of the enteron extend to the hind end of 

 the body below the primitive streak, and the anus not being 

 terminal (Parti, fig. 25), there is at first a postanal gut. This state 

 of things continues (PL XXXVI, figs. 28, 29) until the formation 

 of the proctodseum, which happens when the anus has shifted to 

 the hind end of the body, and the embryo has acquired its full 

 complement of somites (PI. XXXVI, fig. 30). The proctodaeum 

 is due to the growth of the intermediate nuclei. It eventually be- 

 comes of considerable extent, acquires a mesodermal investment 

 from the splanchnic walls of the adjoining somites, and finally 

 constitutes the rectum of the adult (PI. XXXVII, fig. 42). 



The nuclei of the embryonic endoderm of Peripatus are 

 remarkable for being branched and angular. Nuclei of a 

 similar character are found in other animals. Ley dig (No. 19) 

 has described branched nuclei in the Malpighian tubules and 

 epithelium of the alimentary canal of Arthropoda, and Balfour 

 (No. 3) speaks of large angular nuclei as occurring in the 

 yolk-segment of Araneina. The angular shape is not 

 retained in the adult Peripatus. 



The Mesoderm. 



The early stages in the formation of the mesoderm, up to 

 the end of Stage a, have been fully described in Part II of this 

 series. 



The nuclei of the mesoderm, which arise from the nuclei of 

 the primitive streak, extend laterally and grow forward on each 

 side of the blastopore, at a little distance from it, as the 

 lateral mesoblastic bands. The mesoblastic bands are there- 

 fore, primarily at any rate, outgrowths of the lateral portions 

 of the primitive streak nuclei. This is shown clearly by 

 fig. 26 a, b, c of PL XIV, Part II. In this embryo (Stage a) 

 the mesoblastic bands [mb.) had only a very small extension for- 

 wards (five sections in front of the blastopore). They consist of 



