Formation of Archenteron and Metenteron 25 



other embryologists have asserted, because the two phases are 

 quite distinct, the first being the real gastrulation, the effect of 

 protogenesis, while the second so-called phase of gastrulation is 

 a totally different phenomenon, namely the growth in length of 

 the already formed gastrula by accession of material from behind, 

 so that a spherical form is converted into a cylinder. 



In all the Anamnia the rule is that the gastrula cavity or 

 archenteron is formed with an opening to the exterior the 

 blastopore. In all the Anamnia the form of the deuterogenetic 

 proliferating area tends to be annular and the blastopore is the 

 hole in this ring. 



Conditions in Amniota. 



In the Amniota we find a very different state of affairs. 



It is extremely doubtful if the part of the gut cavity which 

 I have termed archenteron (Fig. 13), as distinct from metenteron 

 ever forms in connection with a blastopore. 



An opening between it and the exterior may occur later but 

 that is of a different character and it is really not an archenteric, 

 but a metenteric opening. These two openings are what Brachet 

 calls blastopore virtuel and blastopore reel, Fig. 13. 



The metenteric opening or notopore of some authors (blastopore 

 reel) (of which the more dorsal part becomes neurenteric canal for 

 a while) subsequently disappears, and the ventral part becomes 

 the anus. 



In the Amniota the archenteron always arises as a split either 

 between cells, or else as vacuoles among cells which will be 

 endoderm cells; and in most cases it does riot arise so much as 

 the result of the geometrical effect of segmentation (as in 

 Amphioxus or the Frog), Fig. 6, as by internal secretion of fluid, 

 Fig. 15 C. 



There is only one exception to this, and this a very obvious 

 exception indeed, namely, the segmentation of the Marsupial 

 mammals. But the process in this case is very different from 

 that of any other vertebrate, though at the same time it may be 

 said that it can be derived fairly easily from a meroblastic type 

 of egg similar to that of Prototheria or Sauropsida. 



