THE ORIENTATION OF THE FROG's EGG. 397 



tilised normal egg seems to be such as not to allow a secondary 

 orientation described by Roux. While^ therefore, we cannot 

 deny or refute Roux's statement at present, it seems to me 

 that this point must be carefully examined by other workers 

 before its acceptance will be possible. 



In 1888 Roux records the results of new experiments "with 

 improved methods" to determine the relationship of the embryo 

 to egg-axes. If the blastula were injured at the apex of the 

 black pole the defect was found on the ventral side of the 

 embryo. Roux says that he had previously found that if the 

 blastula was stuck at the equator on the blastopore side the 

 defect appeared in the middle of the medullary folds, and he 

 had concluded that the head half of the embryo was formed in 

 the upper half of the egg, i. e. the embryo was placed vertically. 

 The researches of this year (1888) show that this defect was 

 not a primary phenomenon, but that it represented a later 

 change where a ''reparation" had taken place. 



Roux injured the first anlage of the dorsal lip of the blasto- 

 pore, and found the defect to lie in the cross-connection at the 

 anterior end of the medullary folds. Injury to the blastula or 

 young gastrula at one side of the equator produced a defect in 

 the middle of the medullary folds. Injury to the young gas- 

 trula at a point opposite the gastrula crescent produced defects 

 in the caudal region. Injuries in the middle of the white area 

 gave no defect in later embryo. 



These experiments of Roux's are of great importance, for 

 if true they show the method by which we must regard the blas- 

 topore to be closed. I shall return to this in the final section 

 when speaking of the general problem of concrescence. 



Roux concludes that the embryo lies over the lower hemi- 

 sphere, and that the dorsal lip of the blastopore moves through 

 170°. His figures (' Anat. Anzeiger,' 1888) show the head 

 end of the embryo near that point of the equator at which the 

 embryo first appeared. The anterior connection of the medul- 

 lary folds lies just above the equator upon the black hemi- 

 sphere. From this region the embryo stretches over the lower 

 pole for 170°. 



