BODY CAVITY 



559 



are regular longitudinal folds in Myxine, cross-folds traversing these 

 would form crypts, which may be exaggerated into the pyloric caeca of 

 Teleosteans and Ganoids, while other modifications would give rise to 

 " spiral valves " and the like. In the same way it may be suggested 

 that the numerous important outgrowths of the mid-gut, such as lungs, 

 liver, pancreas, and allantois, so thoroughly justified by their usefulness, 

 may at first have been due to necessary conditions of growth — to the 

 high nutrition, rapid growth, and 

 rapid multiplication of the endoderm. 

 It may be noted that in the develop- 

 ment of the Amphibian Necturus. 

 there are hints of more numerous 

 endodermic diverticula (Piatt). It is 

 also said that the hypochorda — a 

 transitory structure — arising below 

 and subsequent to the notochord, is 

 in part due to a series of dorsal 

 outgrowths from the gut (Stohr). 

 Even the notochord, which arises as 

 a median dorsal fold, may be specula- 

 tively compared to a typhlosole — 

 folded outwards instead of inwards. 

 The future elaboration of the organs 

 which arise as outgrowths of the gut 

 would, however, depend on many 

 factors, such as their correlation with 

 other parts of the body, and would 

 at each step be affected as usual by 

 natural selection. 



Body cavity. — In Amphi- 

 oxiis the ccElom arises as 

 pouches from the archenteron 

 {enterocoelic). In the other 

 Vertebrates, owing to modified 

 processes of development, prob- 

 ably first arising from the 

 presence of much yolk, solid 

 cell masses grow out in place 



Fig. 317. — Section through 

 Elasmobranch embryo. — 

 After Ziegler. 



N.C., Nerve-cord; N., notochord; 

 AO., aorta; G., gut; V., sub- 

 intestinal vein ; M.C., a free 

 mesenchyme cell ; F., beginning 

 of a paired fin ; C, coelom ; U., 

 segmental duct ; M.-M., myo- 

 tome ; M.P., muscle plate ; SK., 

 skeletogenous cells around noto- 

 chord ; EC, ectoderm. 



of hollow sacs, but the cavities 

 which appear later, apparently by splitting of the cell 

 mass (schizoccelic) , are in reality the retarded cavities of true 

 coelom-pouches. A dorsal segmented portion (protoverte- 

 bras) becomes separated off from a ventral unsegmented 

 portion (Fig. 317). It is this ventral portion which forms 

 the body cavity of the adult. In the adult it is divided into 

 gn anterior pericardial and a posterior peritoneal portion. 



