Scientific Proceedings. 



(123) 59 



material is totipotent and an embryo may develop anywhere in the 

 egg, appearing in the less injured regions ; or the material for 

 normal and abnormal development is the same and becomes car- 

 ried downward, during the early stage of normal development, 

 from the upper into the lower hemisphere. 



The author tested these alternatives in two ways. In the first 

 place he removed with a needle the two anterior, or the two pos- 

 terior, or even all four of the upper blastomeres at the eight cell 

 stage. The results showed that when the two upper anterior 

 blastomeres are removed, the head end of the embryo is defective ; 

 when the two upper posterior blastomeres are removed, the pos- 

 terior end sometimes shows defects. When all four of the upper 

 blastomeres are removed, no embryo develops, although the blasto- 

 poric rim may appear near the equator of the egg, the gastrulation 

 process may begin, and the differentiation of the germ layers takes 

 place to a certain extent. 



The author concludes from these results that some at least of 

 the material that goes to form the embryo, lies at first high up in 

 the upper hemisphere of the egg. In the light of this conclusion, 

 it became necessaiy to examine once more the early development, 

 especially the pregastrula stages ; for despite the fact that the frog 

 has been a classic object of study with embryologists for over a 

 hundred years, no one has suspected that the embryo-forming 

 material lies in the upper hemisphere and is transported to the 

 lower hemisphere before the lips of the blastopore have appeared. 



Briefly, the author's examination showed that throughout the 

 early period of segmentation the material of the upper hemisphere 

 gets pushed far out to the sides of the egg. This is brought about 

 largely by the development of the enormous segmentation cavity. 

 During the later cleavage period, the yolk cells of the lower hemi- 

 sphere push upward into the segmentation cavity, almost obliter- 

 ating it. This upward movement of the cells in the interior is 

 compensated for by the moving downward below the equator of 

 the outer layers of the egg. In this way the embryo-forming 

 material is carried into the lower hemisphere. Along its edge the 

 lips of the blastopore develop. The dorsal, lateral and ventral 

 lips roll over the yolk (or more accurately, the yolks draw in be- 

 neath their advancing lips), and the dorsal organs of the embryo 



