294 



EMBRYOLOGY. 



of the previously described process of folding, by means of which 

 the body is formed from the flattened-out germ-layers, the region 

 which originally lies behind the primitive groove comes to lie ventral 

 to and in front of the tail-end. At a somewhat later stage than that 

 shown in fig. 160, the primitive streak in front of the anal membrane 

 grows outward as a small ridge and subsequently enlarges into the 

 tail of the Mammal. The neurenteric canal, located in the ridge, is 

 overgrown by the medullary folds, and upon the complete closure 

 of the latter is incorporated in the neural tube, as in the case of the 

 remaining Vertebrates. In the case of Mammals also there is formed 



a small caudal gut, which sub- 

 sequently degenerates. The more 

 the caudal bud protrudes outward 

 (fig. 161 sch), the more it projects 

 over and beyond the anal mem 

 brane (afm), which constantly 

 moves farther toward the ventral 

 side of the body and is now found 

 between the base of the tail (sch} 

 and the fundament of the allan- 

 tois (al). The rupture of the anal 

 membrane takes place relatively 

 late ; in the case of Ruminants, 

 for example, in embryos that are 

 more than twenty-four days old. 

 Apparently the anus in Birds 

 arises in a manner similar to 

 that in Mammals. According to 



the statements of GASSER and KOLLIKER its opening, produced by 

 the rupture of the anal membrane, occurs on the fifteenth day. 



al 



Fig. 161. Sagittal section through the tail- 

 end of an embryo Sheep 18 days old and 

 with 23 pairs of primitive segments, after 

 BONNET. 



sch, Tail-bud or terminal ridge ; am, amnion ; 

 mk 1 , its mesodermal (somatic) layer ; afm, 

 anal membrane lying ventral to and in 

 front of the tail-bud ; al, allantois. 



It is asserted for many Vertebrates (Petromyzon, Triton, Salamandra, Kana 

 temporaria, Alytes) that the primitive mouth is converted directly into the anus 

 (GASSER, JOHNSON, SEDGWICK, SPENCER, KUPFFER, GOETTE). But since the 

 development of the posterior part of the body proceeds from the margins of 

 the primitive mouth (formation of the chorda and of the middle germ-layer), 

 it would be difficult to understand how, in these cases, the tail-end of the body 

 and a tail-gut could still be formed. Other investigators (SCHANZ and BONNET) 

 find that the primitive mouth is divided into two openings an anterior, which 

 is incorporated in the hind end of the neural canal (canalis neurentericus, 

 chorda-blastopore), and a posterior, which becomes the anus (anal blastopore, 

 anal canal). The statements, which are still contradictory, must be cleared 

 up by means of comparative investigations. 



