14 MOEPHOLOaT OF 



the anterior wall of the gut (Stage E to F) runs at right angles 

 to the long axis of the body. As will afterwards appear this 

 feature is of importance. In longitudinal section^ a commenc- 

 ing differentiation between the branchial and digestive region 

 is perceptible. The cells of the former are columnarj while 

 those of the latter have irregular amoeboid processes which 

 give the inner wall of the gut an irregular contour. The anus 

 is not yet formed. 



In the front end of the third segment of the larva (Stage F), 

 anterior to the ring of cilia, the sides of the gut give rise to a 

 pair of dorso-lateral evaginations; these pouches are the first 

 indications of the gills. No change has occurred in the skin 

 covering them. Subsequently they come in contact with the 

 skin, the walls fuse and then a perforation is formed through 

 the fused portion, apparently occurring by a process of degene- 

 ration of the tissue. Fig. 29 is from a section taken through 

 the side of one of these evaginations. The subsequent appear- 

 ances are shown in figs. 42 and 43, which are from an older 

 larva. 



Notochord. — In the later stages of F and G in the 

 anterior dorsal wall of the gut, arises a most remarkable struc- 

 ture. For reasons which will appear when its later develop- 

 ment and fate is considered, I propose to compare this organ 

 with the notochord of the Chordata, and by this name it will 

 be subsequently spoken of. 



In Stage E it was stated that the anterior wall of the hypo- 

 blast came vertically to join the skin at the mouth (figs. 7 and 

 10). As, however, development proceeds, the dorsal wall of 

 the pharynx becomes partly constricted from the remainder 

 (figs. 20 and 22). As this process of separation of the dorsal 

 wall proceeds, the part so separated grows forwards so that it 

 comes to project slightly in front of the anterior end of the gut 

 (fig. 30). By this means a hypoblastic tube is formed dorsal 

 to the gut, with a lumen which opens into the archenteric 

 cavity (figs. 21, 23, and 30). 



Mesoblast. — The lining of the anterior body cavity in 

 Stages E and F is composed of rounded cells arranged in con- 



