315 



crescent [Morgan ^)] on the surface. The latter appears, however, 

 in the unfertilized egg after 24 hours, if the eggs are allowed to stand 

 quietly. The grey area appears on that part of the egg that stands 

 uppermost. Usually it appears in the region between the black and 

 the white area, but I have seen it appear even at the white pole if 

 this happened to be the highest point of the egg. Eggs that were 

 rotated, as described above, did not show the grey area after 24 hours 

 on any part of the egg. These eggs were taken from the full uterus 

 of a female which had been caught paired with a male, but the eggs 

 could not be fertilized; no doubt because the spermatozoa were not 

 good. They did not move when taken from the testis. In consequence 

 I could not carry out the rotation experiment on the frog's egg. 



A number of other experiments that I have made have shown 

 that, as is now well etablished, the dorsal lip of the blastopore appears 

 on the side of the egg that contains the grey area. Does this grey 

 region develop around a pre-organized region, or is it determined by 

 the action of gravity as claimed by Moszkowski? At present we can 

 not answer this question decisively. In some bunches collected under 

 natural conditions I have observed a well marked grey area ; in others 

 1 have failed to find any at all. In some eggs the grey area is not 

 bisected by the first plane of cleavage, and it may even be bisected 

 by the second cleavage plane. Not infrequently its median plane may 

 lie in the middle of one of the first four blastomeres, as described for 

 this egg by Morgan and Tsuda ^). Whether the plane of symmetry of 

 the embryo follows the plane of symmetry of the grey area (white 

 crescent) remains to be determined. 



It may be questioned whether we can apply the results of the 

 experiments on the toad's egg to the frog. The same experiments 

 should, of course, be made with the frog's egg; but since the two 

 kinds of eggs are so similar in all respects there is great probability 

 that they are similar in this one also. 



The critical points that now remain to be determined are: First, 

 does the point of entrance of the spermatozoon determine the bilaterality 

 of the egg, as claimed by Roux; or. Second, does the grey crescent 

 develop in a pre - organized part of the egg, and if so ' does the egg 

 rotate after fertilization so that this part turns uppermost; or. Third, 

 does the grey crescent appear at any point on the egg that happens 

 to lie uppermost. However this may be, the results on the toad's egg 



1) Morgan, The Development of the Frog's Egg, 1897. 



2) Morgan and Tslda, Quart. Jour. Micr. Science, Vol. 35, 1893. 



