452 DEVELOPMENT OF ELASMOBRA.NCH FISHES. 



the cloaca, and are only visible in the hindermost of my three 

 sections. 



All the structures of the adult cloaca appear to be already 

 constituted by stage O, and the subsequent changes, so far as I 

 have investigated them, may be dealt with in very few words. 

 The perforation of the cloacal involution is carried slowly for- 

 wards, so that the opening into the cloaca, though retaining 

 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 O its anterior 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 1 , 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 O, 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 

 hindermost 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 Fowl 2 . The abdominal pockets, as might be anti- 

 cipated from their structure in the adult, are simple involutions 

 of the epiblast. 



The thyroid body. 



The earliest trace of the thyroid body has come under 

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



1 This Edition, No. vn. p. 152. 



3 Vide Gasser, Entwicklungsgeschichte der Allantois, etc. 



