48 A. Ostroumoff 



Au dem hinteren Rande der Kloakalnaht ist die Theiluug des 

 Schwanzdarmes in zwei Abschnitte, die Kloake und den von allen 

 Seiten von Mesoderm umschlossenen Postaualdarm, zu bemerken 

 (Fig. 6). 



Aus dem Obigen ist leicht zu ersehen, dass die Kloake sich auf 

 Kosten des vorderen Theiles des Schwanzdarmes bildet und die Kloa- 

 kalnaht denjenigen Punkt andeutet, wo sich der Embryonalafter oder 

 Blastoneuroporus befindet und wo später der definitive After auftritt. 



Balfour schreibt über die Entwicklung des definitiven 

 Afters bei den Selachiern Folgendes i. 



»The point where the anus, or more precisely the external ope- 

 uiug 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 PL VI 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 externa! 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. Oppo- 

 site the anus the alimentary canal dilates and forms the cloaca. 



Duriug 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 PI. XI fig. 9(7« 



» Nothing very wortby of note takes place in connection with 

 the cloaca tili stage 0. By this stage we have three important 

 structures developed. (1) An Involution from the exterior to form 

 the mouth of the cloaca or anus. (2) A Perforation leading into the 

 cloaca at the bind end of this. (3) The rudiments of the abdominal 

 pockets. All of these structures are shewn in PL XVIII fig. \a. 



The mouth of the cloaca is formed by an involution of the 

 skin, which is deepest in front and becomes very shallow behind 

 (PL XYIII, fig. 1«, \b). At first only the mucous layerof the skin takes 

 part in it, but when the involution forms a true groove, both layers 

 of the skin serve to line it. At its posterior part, where it is shallow- 

 est, there is present, at stage 0, a slit-like lougitudinal Perfora- 

 tion, leading into the posterior part of the cloaca (PL XVIII fig. Ic) 



1 Development of Elasmobranch Fishes. Loudou ISTS pag. 221 — 223. 



