GEOWTH IN LENGTH OP THE FROG EMBEYO. 237 



that the lower pole was only slightly lighter in colour than the 

 upper, and this for an area not greater in extent than that sub- 

 tended by an angle of 50°. Yet in these the blastopore, though 

 very dark, was quite as light as the lightest part of the unseg- 

 raented ovum. 



If the centre of the blastopore at the moment it is in the 

 stage represented in fig. 20 is not either the lower pole, or 

 some spot extremely close to the lower pole, of the unsegmented 

 ovum, the intense whiteness of this spot must have been pro- 

 duced by a disappearance of pigment previously existing. Is 

 there any evidence of this ? I cannot think of any. On the 

 contrary, there is evidence of increasing pigmentation, as, for 

 instance, in the epiblast-cells as they form upon the sur- 

 face, in the cells of the splitting archenteron, and even in the 

 white cells themselves. If the final position of the blastopore 

 is, as Eoux supposes, at the equator, and at a spot removed 170° 

 from the spot of the first commencement of the blastopore, 

 surely the uncovered part of the surface would be dark, if not 

 black, and certainly not intensely white. 



At the moment the definite outline of the ventral lip of the 

 blastopore is formed the anus of Rusconi thus fashioned is 

 not of uniform tint. The dorsal part of the area is much 

 lighter in colour than the ventral part, fig. 19. When, how- 

 ever, it has diminished to the condition of fig. 20, it is of 

 uniform tint and intensely white. This is accounted for by the 

 more rapid closure of the ventral lip from this moment, as my 

 experiments and those of Morgan and Ume Tsuda demonstrate, 

 as also may be seen by examination of sections as described in 

 a former paper (Robinson and Assheton). 



Except for a slight advance of the dorsal lip of the blasto- 

 pore my experiments do not support Roux^s. 



According to Roux, injuries made at the point x in fig. 5 a 

 ought to have appeared upon the dorsal side in the medullary 

 folds a little way anterior to the blastopore. Instead of which 

 one was in the median line ventral to the blastopore, 

 one was laterally placed on a level with the blastopore, and one 

 laterally placed but far forwards. So, again, if the lips of the 



