320 THE EVOLUTION OF MAH. 



from the intestinal-fibrous layer, at the point where this 

 curves into the outer lamina of the side-layer, into the 

 skin-fibrous layer (Plate IV. Fig. 5, g). The curving-point 

 was distinguished as the middle-plate (Fig. 99, mp). The 

 mesentery is, at first, very short (Plate V. Fig. 14,^) ; but it 

 soon lengthens considerably at the central part of the intes- 

 tinal canal, and takes the form of a thin, transparent, 

 membranous plate, which has to be the more extended the 

 further the folds of the intestine diverge from the place 

 where they are first attached to the vertebral column. The 

 blood-vessels, lymphatic vessels, and nerves which enter 

 the intestinal canal traverse this mesentery. 



Although, therefore, the intestinal canal; in the adult 

 human being forms an extremely complex organ, and 

 though it shows in its details so many intricate and delicate 

 structural arrangements, — into which we cannot enter 

 here, — this entire structure has developed, historically, 

 from that simplest form of primitive intestine which 

 was possessed by our gastrsead ancestors, and which the 

 extant gastrula now exhibits. We have already shown (in 

 Chapter YIII.) that the peculiar Hood-gastrula {Arajphi- 

 gastrula) of Mammals (Fig. 277) may be referred back 

 to the original Bell-gastrula (Archigastrtda) form, which, 

 among Vertebrates, is now accurately retained solely by 

 the Amphioxus (Fig. 276 ; Plate X. Fig. 10). 



Like the latter, the gastrula of Man and of all Mam- 

 mals must be regarded as the ontogenetic reproduction 

 of that phylogenetic evolution-form which we call the 

 Gastrsea, and in which the whole body of the animal is 

 intestine. 



The peculiar form and mode in which the complex 



