192 EMBBYOLOGY OF THE LOWEK VEETEBEATES oh. 



portions each oi' which continued to open into the gut cavity at its 

 hinder end. 



Eectal Gland. — This organ, which occurs in Elasmobranchs, 

 arises as a simple pocket-like outgrowth of the gut-wall. The super- 

 ficially similar caecum of Lung-fishes will be dealt with in con- 

 nexion with the renal organs. 



Cloaca. — In the more archaic Vertebrates the ducts of the 

 excretory organs open into the terminal part of the intestine which 

 is thus a cloaca. It is believed by many that the excretory ducts 

 originally opened at the hind end of the trunk independently 

 of the alimentary canal and it is natural to suppose that the 

 openings of the ducts have become gradually shifted first into close 

 proximity to the anus and finally on to the lining wall of the ali- 

 mentary canal. This again suggests that the cloaca may really be a 

 proctodaeum — that the skin has been involuted to form its lining 

 and that with this involution the renal openings have also been 

 carried inwards. 



Unfortunately the facts of ontogenetic development do not so far 

 as can be seen at present fit this simple and attractive hypothesis. 

 The cloaca is, except for a small portion close to its opening, of 

 purely endodermal origin — the renal ducts open on what is part of 

 the primary enteric wall. A suggested explanation of this fact 

 differing from that mentioned above will be found in the chapter 

 dealing with the renal organs. 



A cloaca seems always to be developed though in some cases 

 (e.g. Teleostean fishes) it flattens out and disappears later so that 

 the renal organs and the gut come to have independent external 

 openings. 



The bursa Fabricii, a conspicuous glandular appendage of the 

 dorsal wall of the cloaca in young birds, has usually been regarded as 

 proctodaeal in its origin but it is now known to arise in ontogeny 

 from vacuolar spaces in a solid projection from the cloacal rudiment, 

 dorsal to the stalk of the allantois (Wenckebach, 1888) and would 

 therefore appear to belong to the mesenteron rather than to the 

 proctodaeum. 



The anal opening of the Vertebrate, as may have been gathered 

 from Chap. II., is to be regarded as representing morphologically a 

 portion of the gastrular mouth or protostoma. In a large number 

 of Vertebrates however the opening arises in ontogeny not in this 

 way but rather as a secondary perforation, although even in such 

 cases the perforation arises in the line of the closed protostoma. 



Temporary Occlusion of the Alimentary Canal.. — The ali- 

 mentary canal is, in correlation with its function, a hollow tube. 

 In a large number of Vertebrates, however, there are more or less 

 extended periods of development during which the cavity is com- 

 pletely absent, either throughout the length of the canal or in 

 certain portions. 



In its simplest condition this occurs as a special case of the 



