DEVELOPMENT OF ELASMOBRANCH FISHES. 223 



words. The perforation of the cloacal involution is carried 

 slowly forwards, so that the opening into the cloaca, though re- 

 taining its slit- like character, becomes continuously longer ; by 

 stage Q its size is very considerable. The cloacal involution, 

 relatively to the cloaca, recedes backwards. In stage its an- 

 terior end is situated some distance in front of the opening of 

 the segmental duct into the cloaca ; by stage P the front end of 

 the cloacal involution is nearly opposite this opening, and by 

 stage Q is situated behind it. 



As I have shewn elsewhere^ the so-called abdominal pores 

 of Scyllium are simple pockets open to the exterior, but without 

 any communication with the body-cavity. By stage Q they are 

 considerably deeper than in stage 0, and retain their original 

 position near the hind end of the opening into the cloaca. The 

 opening of the urinogenital ducts into the cloaca will be 

 described in the section devoted to the urinogenital system. 



In Elasmobranchs, as in other Vertebrata, that part of the 

 cloaca which receives the urinogenital ducts, is in reality the 

 hinder most section of the gut and not the involution of epiblast 

 which eventually meets this. Thus the urinogenital ducts at 

 first open into the alimentary canal and not to the exterior. 

 This fact is certainly surprising, and its meaning is not quite 

 clear to me. 



The very late appearance of the anus may be noticed as a 

 point in which Elasmobranchs agree with other Vertebrata, 

 notably the FowP. The abdominal pockets, as might be anti- 

 cipated from their structure in the adult, are simple involutions 

 of the epiblast. 



The thyroid hochj. 



The earliest trace of the thyroid body has come under 

 my notice in a Torpedo embryo slightly older than I. In this 

 embryo it appeared as a diverticulum from the ventral surface 

 of the throat in the region of the mandibular arch, and extended 

 from the border of the mouth to the point where the ventral 

 aorta divided into the two aortic branches of the mandibular 



1 This Journal, Vol. x. p. 34. 



^ Vide Gasser, Entwicklungsgeschlchte der AUantois, etc. 



