THE ORIENTATION OF THE FROG's EGG. 401 



along the border line between the black and white hemispheres. 

 These lateral medullary folds can be traced for only a short 

 distance. 



Between the anterior connection of the medullary folds and 

 the white is found a^sraall plate of ectodermal cells. The same 

 embryo ^seen from below is shown in fig. xv. We see that the 

 extent of white exposed corresponds to the whole of the lower 

 white area, — in fact, to somewhat more of the lower hemisphere 

 than would be finally enclosed by the normal blastopore; for 

 in the normal egg the far edge of the white, where it shades off 

 into the black, does not normally become involved in the 

 closure of the blastopore. This was shown definitely in the 

 experiments made by sticking the lower pole, and is corrobo- 

 rated by the fact, that in these abnormal embryos the far side 

 of the large blastopore contains much more pigment than 

 does the ventral surface of the yolk-plug of the normal embryo. 

 Hence any statement as to the extent of the white closed over 

 by normal embryos, based on these abnormal embryos, will give 

 an erroneous conclusion. This, I believe, Roux has drawn. 



The embryos produced in the salt solution may be examined 

 at each stage of their development, and the exact method by 

 which the blastopore forms be followed out. This gives a de- 

 cided advantage over the haphazard finding of embryos already 

 formed. In watching such embryos one sees that the blasto- 

 pore extends from its point of origin differently in these em- 

 bryos than in normal embryos. Instead of the corners of 

 the blastopore extending downwards to produce a deep cres- 

 cent or horseshoe-shaped outline, they extend laterally around 

 the border line between black and white. Hence results, I 

 believe, a more extensive enclosure of the lower hemisphere 

 than under normal conditions. 



In fig. XVI is drawn another embryo, differing from the last 

 only in the| greater extent of the medullary plate lying in the 

 black hemisphere. This is due without doubt to the greater 

 extent to which the dorsal^^lip of the blastopore has crept over 

 the white.2 [Tigs, xvii and xviii are drawings of embryos 

 where the^overgrowth of the dorsal lip hasbeen carried farther 



