The Development of the Frog 25 



extending into the dorsal lip of the blastopore. 

 This process may be roughly illustrated, per- 

 haps, by placing a rubber band around the 

 equator of a ball, and then gradually slipping 

 two opposite sides of the band towards one 

 pole of the ball, until they meet and form a 

 single broad band lying in a meridional instead 

 of in an equatorial position. 



This process of concrescence is difficult 

 of determination, and it will probably not be 

 practicable for students to work it out in the 

 laboratory. 



As has been said, the growth of the ectoblast 

 over the yolk-cells takes place much more 

 rapidly from the dorsal lip of the circular 

 groove, which we have called the blastopore, 

 so that, while this groove never becomes very 

 deep on the lower side, on the upper side it 

 becomes a long, narrow slit extending nearly 

 to the opposite side of the embryo (Fig. 9, 

 MN}. This slit, whatever may be the exact 

 method of its formation, is the primitive di- 

 gestive tract of the frog, and is known as the 

 mesenteron or archenteron. It is much wider 

 from side to side than it is in a dorso-ventral 

 direction ; and while its roof is made up of a 

 more or less clearly defined layer of closely 



