108 FORMATION OF THE ANUS. 



pletely adhere (4th month and onwards) ; and this causes the pancreas to appear to lie alto- 

 gether behind the peritoneal cavity, in place of being situated between the two layers of the 

 mesogastrium as is at first the case in the human embryo, and as is frequently found in other 

 mammals during life. 



The free or floating part of the great omentum is formed by an extension of that part of 

 the omental fold which turns upwards towards the greater curvature of the stomach from 

 the surface of the transverse colon. The fold is at first clearly double, and in some animals 

 remains so, but in man its two layers coalesce a little while after birth, and after a year or 

 two can no longer be separated. It extends gradually, first over the transverse colon (third 

 month), later over the coils of the small intestine. 



The spleen becomes formed within the substance of the mesogastrium (fig. 

 127, spl). It is developed wholly from mesoblast, and in close connection with the 

 pancreas. It appears during the second month in the human embryo, and grows 

 slowly during fcetal life, the Malpighian corpuscles being the last parts to appear. 



Formation of the anus. The anal invagination of the epiblast, which eventu- 

 ally by absorption of the septum between it and the hypoblast of the hind-gut opens 

 into the alimentary tube, is termed the proctodceum. The junction with the hind- 

 gut occurs at a little distance from the posterior extremity, so that there is a blindly 

 terminating post-anal or subcaudal portion of the gut beyond the junction with the 

 proctodasum ; this, however, shrinks and disappears even before the absorption 

 of the septum. This part of the hind- gut represents a cloaca, since it receives 

 through the allantois the ducts of the urinary and genital organs. The sepa- 

 ration of the permanent anus from the urogenital orifice, which occurs in all 

 mammals above monotremes, is the result of a later process of development (see 

 p. 127). 



In mammals the actual amount of proctodseal invagination is very small. The 

 septum between the hind-gut and the exterior (anal membrane) is throughout formed 



Fig. 130. LONGITUDINAL SECTION THROUGH THE POSTERIOR END OF A SHEEP'S EMBKYO, SHOWING THE 



ANAL MEMBRANE. (Bonnet.) 



ep, epiblast ; hy. hypoblast ; mes, mesoblast ; h.g, hinclgut ; am, amnion ; an, anal membrane ; 

 p. s, primitive streak ; all, allantois-rudiment. 



by two epithelial layers only, viz., hypoblast and epiblast, which here are in contact 

 with one another without the intervention of mesoblast (fig. 130, an). This con- 

 dition of juxtaposition of the two layers is in fact directly derived from the union 

 of the two layers which occurs at the primitive streak and groove, and if the latter 

 be looked upon as representing the blastopore, the anus may in a sense be considered 

 to be formed from a part of that aperture. In some lower vertebrates the anus has 

 been shown to be directly produced from the blastopore. 



