TlfE ORGANS OF THIi I M LAYKU. 



293 



around clor-.illy into the neural tube as the neurenteric canal. The 

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



The post :m:il init, sooner or later, undergoes regressive metanu>r- 

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

 thelial cord, afterwards deiaches itself from the ana! part .f the 

 intestine and from the neural tube, and then disappea; -her. 



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, KO'LLIKER, BONNKT. K 1:11:1.!. and GtiAOOMm, 

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



Fig. 160. - Sagittal section through the posterior end of an embryo Sheep 16 days old and with 

 5 pairs of primitive segments, after BONNI.T. 



al, Allantois ; afm, anal membrane ; am, anmion ; ah, amuiotic cavity ; ak, outer germ-layer, and 

 mi- 1 , middle germ-layer, which share in the formation of the amnion ; np, neural plat* as 

 it merges into the primitive streak ; pr, primitive groove in the region of the neun 

 canal ; U; inner germ-layer ; ifc% 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 

 1 1 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 tho 

 two latter layers always remain separated from each other by a 

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

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

 as deviating from the representation ju.-t 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 



