6Q DEVELOPMENT OF THE PROG'S EGG [Cn. VI 



surface, the final stages of overgrowth are completed. The 

 ventral lip of the blastopore has moved somewhat forward, as 

 previously explained, and this slight forward movement proba- 

 bly takes place by the growth toward the median line of the 

 material at the sides of the ventral lip. 



There are other changes closely bound up with the preceding 

 phenomena and, although these changes take place simultane- 

 ously, it will be necessary first to consider them separately, and 

 then to try to combine tnem into a single statement. The 

 changes involve, 1) the formation .of- the archenteron, 2) the 

 progression of the blastoporic rim oYer the lower hemisphere, 

 3) the origin of the middle layer or mesoderm. 



The Formation of the Archenteron 



1) When the dorsal lip appears, certain cells pull away from 

 the surface, leaving their outer pigmented ends exposed for a time 

 (Fig. 15, D, Fig. 12, H). These cells are near the border-line 

 between the black and white regions, but lie distinctly amongst 

 the white cells. The next change involves the sinking in be- 

 neath the surface of the region in which these cells are present. 

 The dorsal lip of the blastopore now begins its movement over 

 the lower hemisphere. From the surface we can see that the 

 crescent becomes longer and longer, the horns extending out- 

 wards along the black- white border but well within the white. 

 The same changes that took place where the dorsal lip first 

 appeared, now take place also wherever the crescent extends. 

 First certain superficial cells pull into the interior of the egg 

 leaving only their pigmented ends at the surface, and then this 

 area of pigment sinks below the general surface. Simultane- 

 ously the edges of the blastopore roll over the inturned (invagi- 

 nated) cells. The same changes also take place at the posterior 

 or ventral lip of the blastopore, when the two horns of the lat- 

 eral lips have met there. It is necessary to examine sections 

 that have been cut in several planes in order to follow the 

 changes that take place during the further overgrowth of the 

 blastopore. If we examine a median longitudinal (sagittal) 

 section at the time when the dorsal lip has just begun to roll 

 over, we find (Fig. 25, A) that a narrow space is left between 

 the dorsal lip and the surface of the lower hemisphere over 



