THE ORGANS OF THE INNEE GERM-LAYER. 



293 



around dorsally into the neural tube as the neurenteric canal. The 

 primitive mouth, now closed, formerly lay at the place of bending. 



The post-anal gut, sooner or later, undergoes regressive metamor- 

 phosis in all Vertebrates ; it loses its cavity, becomes a solid epi- 

 thelial cord, afterwards detaches itself from the anal part of the 

 intestine and from the neural tube, and then disappears altogether. 

 Thereby the neurenteric canal, the last remnant of the primitive 

 mouth, has ceased to exist. 



A few still more specific statements, in accordance with the repre- 

 sentations of StRAHL, KoLLIKEtl, BONNET, KeIBEL, and GlACOMINI, 

 concerning the formation of the anus in Mammals, may be mentioned 



Tig. 160.— Sagittal section throiigli the posterior end of an embryo Sheep 16 days old and with 

 5 pairs of primitive segments, after Bonnet. 



alj AllantoJB ; Oifm, anal membrane ; am, amnion ; ah, amniotic cavity ; ak, outer germ-layer, and 

 HiJfcS middle germ-layer, which share in the formation of the amnion ; np, neural plate as 

 it merges into the primitive streak ; p}', primitive groove in the region of the neurenteric 

 canal ; ik, inner germ-layer ; mk'', splanchnic portion of the middle germ-layer ; d, alimentary 

 tube. 



in this connection. The first fundament. of the anus is demonstrable 

 even in embryos with few primitive segments. At the posterior end 

 of the primitive streak — at the anterior end of which the neurenteric 

 canal is situated — the anal membrane is formed by the disappearance 

 of the middle germ-layer over a small area and the close contact of 

 entoderm and epidermis. This, however, takes place so that the 

 two lattfer layers always remain separated from each other by a 

 sharp contour (fig. 160 afrn). One might be inclined to regard 

 this position, at the, hindermost end of the primitive streak {pr), 

 as deviating from the representation just given, according to which 

 the anus arises on the ventral side of the body somewhat in front of 

 the neurenteric canal. That is not the case,, however, as the further 

 course of development teaches ; for in meroblastic eggs, in consequence 



