86 DEVELOPMENT OF THE FROG'S EGG [Cii. VIII 



median plane of the body coincide. The exceptions may be 

 due to the rough treatment of the eggs.^ jS^ewport ('51) had 

 previously made a similar experiment on normal eggs, i.e. eggs 

 not fixed artificially, and had reached the same conclusion as 

 Pfliiger, but Newport's results were unknown to Pfliiger when 

 he made liis experiments. 



Pfliiger was led from certain results of his experiments to 

 observe carefully the position of the e^g at the time when the 

 normal embryo was developing. He found, as has been already 

 described, that the dorsal lip of the blastopore appeared in the 

 white below the equator of the egg. He noted in the living 

 Qgg that the blastopore slowly migrated over the white hemi- 

 sphere, and that it finally closed nearly 180 degrees from the 

 point of its first appearance. Subsequently the whole egg slowly 

 rotated, so that the small blastopore traced the same path (but 

 in a reversed direction) over which the dorsal lip of the blasto- 

 pore had passed. The results show that the nervous system 

 develops over the lower wdiite hemisphere of the egg. The 

 material for the nervous system comes from the substance of 

 the lips of the blastopore as they move over and cover the 

 loAver hemisphere. This material, from which the nervous 

 system is formed, is at first somewhat lighter in color than 

 the pigmented hemisphere of the egg. It is darker, however, 

 than the white material of the lower hemisphere. 



If in normal eggs the first cleavage -plane corresponds to the 

 median plane of the body of the embryo, does the same relation 

 hold for eggs that have segmented in abnormal positions ? In 

 other words, does the median plane of the body in eggs that 

 have been turned so that the primary axis is no longer vertical 

 still correspond to one of the primary meridians of the egg., or to 

 one of the secondary (i.e. segmentation) meridians? Pfliiger's 

 observations showed that in eggs with oblique primary axes 

 tile plane of the first cleavage is not identical with the median 

 plane of the embryo, but forms different angles with it. In 

 forty-eight eggs there were thirty-three in which the median 

 plane of the embryo coincided with the lyrimary axis. In the 



1 Or else to a very early rotation of the egg, either as it shifts around its cen- 

 tre of gravity during gastrulation, or from the action of surface-cilia. — T. H. M. 



