222 CLOACA AND ANUS. 



the cloaca is connected with the prseanal section of the ali- 

 mentary canal in front, and the postanal section behind ; the 

 latter, however, by stage L, as has been stated above, atrophies, 

 with the exception of a very small rudiment. In stage L the 

 posterior part of the cloaca is on a level with the hind end 

 of the kidneys, and is situated behind the posterior horns of the 

 body-cavity, which are continued backwards to about the point 

 where the segmental ducts open into the cloaca, and though very 

 small at their termination rapidly increase in size anteriorly. 



Nothing very w^orthy of note takes place in connection 

 with the cloaca till 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 per- 

 foration leading into the cloaca at the hind end of this. 

 (3) The rudiments of the abdominal pockets. All of these 

 structures are shewn in PI. xviii. fig. la, lb, Ic. 



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

 skin, which is deepest in front and becomes very shallow 

 behind (PI. XYIII. fig. la, Ih). At first only the mucous layer 

 of 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 shallowest, there is present, at stage O, 

 a slit-like longitudinal perforation, leading into the posterior 

 part of the cloaca (PI. xviii. fig. Ic) and forming its external 

 opening. Elsewhere the wall of the cloaca and cloacal groove 

 are merely in contact but do not communicate. On each side 

 of the external opening of the cloaca there is present an invo- 

 lution (PI. XVIII. fig. Ic, ah. p.) of the skin, which resembles 

 the median cloacal involution, and forms the rudiment of an 

 abdominal pocket. These two rudiments must not be confused 

 with two similar ones, which are present in all the three sections 

 represented, and mark out the line which separates the limbs 

 from the trunk. These latter are not present in the succeeding 

 stages. The abdominal pockets are only found in sections 

 through the opening into 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 



