514 



ABDOMINAL PORES. 



wall the splauchnic layer. Between the two splanchnic layers is 

 placed the gut. On the ventral side, in the region of the permanent 

 gut, the two halves of the body cavity soon coalesce, the septum be- 

 tween them becoming absorbed, and the splanchnic layers of epithe- 

 lium of the two sides uniting at the ventral side of the gut, and the 

 somatic layers at the median ventral line of the body wall (fig. 851). 

 In the lower Vertebrata the body cavity is originally present even 

 in the post-anal region of the trunk, but usually atrophies early, fre- 

 quently before the two halves coalesce. 



On the dorsal side of the gut the 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 (fig. 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 epithelium becomes in 

 the adult intimately united with a layer 

 of the subjacent connective tissue, and 

 constitutes with it a special lining mem- 

 brane for the body cavity, known as the 

 peritoneal membrane. 



Abdominal pores. In the Cyclosto- 



tnata, the majority of the Elasmobranchii, 

 the Ganoidei, a few Teleostei, the Dipnoi, 

 and some Sauropsida (Chelonia and Croco- 

 dilia) the body cavity is in communication 

 with the exterior by a pair of pores, known 

 as abdominal pores, the external open- 

 ings of which are usually situated in the 

 cloaca'. 



The ontogeny of these pores has as yet 

 been but very slightly investigated. In the 

 Lamprey they are formed as apertures lead- 

 ing from the body cavity into the excretory 

 section of the primitive cloaca. This section 

 would appear from Scott's (No. 87) observa- 

 tions to be derived from part of the hypo- 

 blastic cloacal section of the alimentary 

 tract. 



Fig. 351. Section thbough 

 the trunk of a scyllium embbyo 

 slightly younger than 28 f. 



ap.c. spinal canal ; W. white 

 matter of spinal cord ; pr. poste- 

 rior nerve-roots ; ch. uotochord ; 

 X. sub-notochordal rod; ao. aorta ; 

 mp. muscle-plate ; mp'. inner layer 

 of muscle-plate already converted 

 into muscles ; Vr. rudiment of 

 vertebral body ; st. segmental 

 tube ; sd. segmental duct ; sp.v. 

 spiral valve ; v. subintestinal vein ; 

 p.o. primitive generative cells. 



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

 ' Fori Abdominales of Vertebrata." Journal of Anat. and Physiol. ,^61. xiv., 1879. 



