68 THE CHANGES FROM STAGE A TO F. 



mites (Plate VIII, fig. 32, and Plate VIII, figs. 14, 16 a, 

 &c.). Its further development will be followed subsequently, 

 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 (Plate II, fig. 25), there is at first a postanal gut. This 

 state of things continues (Plate VIII, 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 (Plate VIII, fig. 30). 

 The proctodseum is due to the growth of the intermediate nuclei. 

 It eventually becomes of considerable extent, acquires a meso- 

 dermal investment from the splanchnic walls of the adjoining 

 somites, and finally constitutes the rectum of the adult (Plate 

 IX, 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. Leydig (No. 35) 

 has described branched nuclei in the Malpighian tubules and 

 epithelium of the alimentary canal of Arthropoda, and Balfour 

 (No. 19) speaks of large angular nuclei as occurring in the 

 yolk-segments 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 Chapter II. 



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 Plate V. In this embryo (Stage a) the 



