CLOACA AND ANUS. 451 



During 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 PL n, fig. ga. During the same stage the 

 segmental ducts grow downwards to the cloaca, and open into it 

 in the succeeding stage (PI. n, fig. gfr). Up to stage K the 

 cloaca is connected with the prseanal section of the alimentary 

 canal in front, and the postanal section behind ; the latter, how- 

 ever, 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 worthy of note takes place in connection with 

 the cloaca till stage O. By this stage we have three important 

 structures developed, (i) An involution from the exterior to 

 form the mouth of the cloaca or anus. (2) A perforation 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. 19, figs, i a, \b, ic. 



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

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

 (PL 19, figs, la, id). 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 (PL 19, fig. ic) and forming its external opening. Else- 

 where 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 involution (PL 19, fig. 

 ic, ab. p.) of the skin, which resembles the median cloacal involu- 

 tion, 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 



