DEVELOPMENT OF ELASMOBRANCH FISHES. 221 



it communicate with the neural canal. It was visible in one 

 series for about 40 sections, and was continued forwards by a 

 few granular cells, lying between the aorta and the caudal vein. 

 The portion of the postanal section of the alimentary tract just 

 behind the cloaca, was in the same embryo represented by a 

 still smaller rudiment of the dilated part which at an earlier 

 period opened into the cloaca. 



Later than stage L no trace of the postanal section of the 

 alimentary canal has come under my notice, and I conclude that 

 it vanishes without becoming converted into any organ in the 

 adult. Since my preliminary account of the development of 

 Elasmobranch Fishes was written, no fresh light appears to 

 have been thrown on the question of the postanal section of the 

 alimentary canal being represented in higher Yertebrata by the 

 allantois. 



The cloaca and anus. 



Elasmobranch s agree closely with other Vertebrates in the 

 formation of the cloaca and anus, and in the relations of the 

 cloaca to the urinogenital ducts. 



The point where the anus, or more precisely the external 

 opening of the cloaca, will be formed, becomes very early 

 marked out by the approximation of the wall of the alimen- 

 tary tract and external skin. This is shewn for stages H and I 

 in PL Yi. an. 



Between stages I and K the alimentary canal on either 

 side of this point, which we may for brevity speak of as the 

 anus, is far removed from the external skin, but at the anus 

 itself the lining of the alimentary canal and the skin are in 

 absolute contact. There is, however, no involution from the 

 exterior, but, on the contrary, the position of the anus is 

 marked by a distinct prominence. Opposite the anus the 

 alimentary canal dilates and forms the cloaca. 



During stage K, just in front of the prominence of the anus, 

 a groove is formed between two downgrowths of the body-wall. 

 This is shewn in PL XI. fig. 9a. During the same stage 

 the segmental ducts grow downwards to the cloaca, and open 

 into it in the succeeding stage (PL xi. fig. 9^). Up to stage K 



