258 



BERTRAM G. SMITH. 



which by common consent are considered to have been their 

 progenitors. 



In the higher animals the gastrula stage is regarded as the 

 embryonic representative of the radial, two-layered ancestral 

 type; the blastopore is the representative of the single opening 

 or so-called mouth of the ccelenterate. 



Bilateral animals have arisen from radiate forms by the elonga- 

 tion of one of the transverse axes of the latter, the oral face be- 

 coming the ventral aspect, and the aboral face the dorsal aspect 

 (this relation is reversed in the vertebrates). The mouth, 

 meanwhile, shifted its position so as to lie near the anterior 

 extremity of the new long axis, and the lateral portions, growing 

 together more or less completely along the region formerly 

 occupied by the mouth, gave rise to the process of concrescence 



in the ontogeny. 



What, then, was the origin of the anus 

 in bilateral animals? In different em- 

 bryos we find that the blastopore gave 

 rise, sometimes to the mouth (through 

 the persistence of the anterior part of 

 the blastopore, as in the earthworm), 

 sometimes to the anus (through the per- 

 sistence of the posterior part of the 

 blastopore, as in many vertebrates), and 

 rarely to both (as in Peripatus, where 

 the elongated blastopore closes in the 

 middle). The only possible interpreta- 

 tion of these facts would seem to be that 

 the blastopore originally gave rise to 

 both mouth and anus. The case of 



Peripatus (Fig. 44) is then an interesting and apparently isolated 

 remnant of the ancestral mode of development, or perhaps a 

 reversion to it. It is interesting to note that in Peripatus the 

 two halves of the body are quite well differentiated independently 

 on the two sides of the blastopore; later, these two halves are 

 brought together. 



Concrescence is then a sequence of the closure of the blasto- 

 pore, which primitively extended the whole length of the embryo, 



FIG. 44. Diagram show- 

 ing the mode of blastopore 

 closure in Peripatus. 



