GROWTH IN LENGTH OF THE FROG EMBRYO. 241 



line ; the dotted line represents only approximately the sup- 

 posed division between the parts of the embryo derived from 

 the primary and secondary areas of proliferation respectively. 

 The subsequent fate of the secondary area of proliferation (or 

 primitive streak) I have, with Dr. Robinson, described and 

 discussed in a former paper. The three figures 8, 9, and 10 

 represent my views of the extent to which the white pole 

 becomes overgrown by the dorsal lip of the blastopore. The 

 neural plate is indicated as in fig. 11 by the continuous line. 

 The extreme anterior end of this part of the epiblast has been 

 obtained by subtracting the amount due to overgrowth from 

 the total amount observed when definitely established. 



It does not necessarily follow that the distance through 

 which the edge of the dorsal lip of the blastopore advances 

 represents the total growth in length due to that part of the 

 secondary area of proliferation. In order to advance, the lip of 

 the blastopore has to exert pressure upon the " yolk plug," 

 causing it to be forced inwards. It thus follows that an equal 

 force must be exerted in the other direction. What effect this 

 has will depend upon the strength of the resistance offered by 

 the anterior wall of the embryo. 



From the fact that the archenteron is a slit in the stage 

 represented by figs. 8, 21, and 17, and is a spacious cavity in 

 the stage represented by figs. 9, 13, and 19, it seems likely 

 that the growth of the blastoporic lip is rendered evident, not 

 only by the amount of white yolk plug covered, but also by the 

 arching up of the dorsal roof of the archenteron. But, on the 

 other hand, the arching up may be in great part, if not entirely, 

 due to its own interstitial growth ; though I do not think this is 

 likely, for it seems to me to require the thrusting energy of 

 the blastoporic lip to account for the obliteration of the seg- 

 mentation cavity. 



If Schultze's idea of the apparent overgrowth of the white 

 yolk plug being due to a rolling inwards of the white pole were 

 correct, ought not the ventral end of the segmentation cavity 

 to become obliterated before the dorsal ? But it is the dorsal 

 part that first disappears. 



VOL. 37, PART 2. NEW SEE. Q 



