45 O DEVELOPMENT OF ELASMOBRANCH FISHES. 



(PI. 1 1, fig. <)d}, and a rupture here occurs in it at a slightly later 

 period. 



In stage L the posterior part of the postanal section of the 

 canal is represented by a small rudiment near the end of the 

 tail. The rudiment no longer has a terminal vesicle, nor does 

 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 Vertebrata by the 

 allantois. 



The cloaca and anus. 



Elasmobranchs 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 alimentary 

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

 PI. 8 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. 



