626 ABDOMINAL PORES. 



two halves of the body cavity never coalesce, but eventually the 

 splanchnic layers of epithelium of the two sides, together with a 

 thin layer of interposed mesoblast, form a delicate membrane, 

 known as the mesentery, which suspends the gut from the dorsal 

 wall of the body (figs. 119 and 351). On the dorsal side the 

 epithelium lining of the body cavity is usually more columnar 

 than elsewhere (fig. 351), and its cells partly form a covering for 

 the generative organs, and partly give rise to the primitive 

 germinal cells. This part of the epithelium is often known as 

 the germinal epithelium. 



Over the greater part of the body cavity the lining epithe- 

 lium becomes in the adult intimately united with a layer of the 

 subjacent connective tissue, and constitutes with it a special 

 lining membrane for the body cavity, known as the peritoneal 

 membrane. 



Abdominal pores. In the Cyclostomata, the majority of the Elas- 

 mobranchii, the Ganoidei, a few Teleostei, the Dipnoi, and some Sauropsida 

 (Chelonia and Crocodilia) the body cavity is in communication with the 

 exterior by a pair of pores, known as abdominal pores, the external 

 openings of which are usually situated in the cloaca 1 . 



The ontogeny of these pores has as yet been but very slightly investigated. 

 In the Lamprey they are formed as apertures leading from the body cavity 

 into the excretory section of the primitive cloaca. This section would 

 appear from Scott's (No. 87) observations to be derived from part of the 

 hypoblastic cloacal section of the alimentary tract. 



In all other cases they are formed in a region which appears to belong 

 to the epiblastic region of the cloaca ; and from my observations on Elas- 

 mobranchs it may be certainly concluded that they are formed there 

 in this group. They may appear as perforations (i) at the apices of 

 papilliform prolongations of the body cavity, or (2) at the ends of cloacal 

 pits directed from the exterior towards the body cavity, or (3) as simple 

 slit-like openings. 



Considering the difference in development between the abdominal pores 

 of most types, and those of the Cyclostomata, it is open to doubt whether 

 these two types of pores are strictly homologous. 



In the Cyclostomata they serve for the passage outwards of the genera- 

 tive products, and they also have this function in some of the few Teleostei 

 in which they are found ; and Gegenbaur and Bridge hold that the primitive 

 mode of exit of the generative products, prior to the development of the 

 Miillerian ducts, was probably by means of these pores. I have elsewhere 



1 For a full account of these structures the reader is referred to T. W. Bridge, 

 "Pori Abdominales of Vertebrata. " Journal of Anat. and Physiol. , Vol. XIV., 1879. 



