68 DEVELOPMENT OF THE FROG'S EGG [Ch. VI 



the blastopore-lips. This statement does not necessarily imply, 

 however, that the segmentation-cavity was prepared especially 

 in view of the subsequent changes. 



It will be seen from the foregoing account that the walls of 

 the archenteron are formed as the blastopore closes in. The 

 floor of the archenteron (Fig. 25, B) is nothing more than the 

 surface of the lower white hemisphere that is overgrown. The 

 origin of the roof and sides of the archenteron is somewhat diffi- 

 cult to understand. We have seen that around the crescent of 

 the blastopore certain cells have pulled in, leaving a depression 

 on the surface. It is impossible to say just how far the cells 

 that pull in continue to be drawn inward, because simultane- 

 ously the lips of the blastopore roll over. This brings us to 

 a discussion of the second topic. 



The Overgrowth of the Blastoporic Rim 



2) There are at least two ways in which we, may think of 

 the closing in of the lips of the blastopore, i.e. there are two 

 ways, either of which might explain the covering of the white 

 by the black cells. We may think of the free edge of the 

 blastopore as growing toward a middle point. Or we may 

 imagine that the lateral and dorsal edges actually roll in 

 toward the' middle line. The latter process seems to be that 

 which probably takes place, for Jordan ('93) has seen the outer 

 dark cells actually rolling over and into the archenteron in the 

 living egg. 



The dorsal and lateral walls of the archenteron will then be 

 formed in part, or entirely, from those cells of the surface 

 that have rolled in and have come to lie beneath the surface. 

 These are the cells, therefore, that have been at one time 

 situated at the surface of the embryonic ring, and inasmuch 

 as the advance of the dorsal lip takes place very largely by 

 the fusion of the lateral lips, it follows that the material for 

 the greater part of the dorsal wall of the archenteron comes 

 from cells at one time on the outer surface of the egg. I am 

 inclined to think that at first there is also an actual in-pulling 

 of cells along the blastoporic rim so that cells at one time below 

 the outer surface come also to stand, later, at the sides of the 

 archenteron, i.e. where the dorsal and ventral walls meet. 



