THE STOMATOD^UM AND PEOCTOD^UM. 149 



of the narial openings, divides the stomatodaeal from the pharyn- 

 geal portion. 



After the mouth opening is established, the lips of the 

 stomatodaeum grow forwards rapidly, and in connection with 

 them the powerful horny jaws of the tadpole, by which it crops 

 its food, are speedily developed (Fig. 65, j). 



The proctodaeum. The mesenteron, from its first appear- 

 ance, and throughout the early stages of development, communi- 

 cates with the exterior through the blastopore (Fig. 60, B). It 

 also communicates, through the neurenteric canal, TS T C, with the 

 central canal of the spinal cord and brain ; this communication 

 persists for some time after the blastopore has closed (Fig. 61)," 

 but is lost when the tail begins to lengthen (Fig. 69). 



The proctodaeal invagination appears as a pit-like depression 

 at the ventral end of the primitive streak (Figs. 58, B, c, D, 

 and 60, PD). In embryos of about 4 mm. length (Fig. 61), this 

 invagination reaches and opens into the rectal portion of the 

 mesenteron, i.e. the portion which lies posterior to the mass of 

 yolk-cells. 



The closure of the blastopore usually occurs before the anal 

 perforation is completed ; but it may happen that the two 

 openings into the mesenteron are present for a time simul- 

 taneously. 



In the frog this proctodeeal invagination is a new opening 

 into the mesenteron, and is not a persistent part of the original 

 communication of the mesenteron with the exterior, through 

 the blastopore. If it be borne in mind, however, that the 

 proctodasal invagination appears in the primitive streak, and 

 as an actual deepening of the ventral end of the primitive 

 groove (Fig. 58, B) ; and further, that the primitive streak is 

 formed by concrescence of the lips of the blastopore, then the 

 formation of the proctodaeal invagination may be viewed, not as 

 an entirely independent depression of the surface, but as a 

 re-opening of the ventral portion of the blastopore. This view 

 is strongly supported by the development of other Amphibians, 

 in some of which the blastopore actually persists as the anus. 



It will be noticed that the proctodaeal, or anal, opening is 

 established some time before the embryo hatches, while the 

 stomatodaeal or mouth opening is not formed until a consider- 



