42 DEVELOPMENT OF THE FROG'S EGG [Ch. IV 



Correspondence of the First Cleavage-plane and thjb 

 Median Plane of the Embryo 



If the egg when in the two-cell stage be fixed 1 so that it can- 

 not rotate in a horizontal plane, and if such an egg be carefully 

 watched until the moment when the medullary folds have just 

 appeared, it will be found that the position of the plane of first 

 cleavage corresponds approximately, or even exactly, to the 

 median plane of the body of the embryo. This experiment 

 was first made by Newport in 1851, subsequently by Pfliigef 

 ('83), and Roux ('85), and later by other workers. 2 If, how- 

 ever, during the subsequent cleavage-periods, i.e. during the 

 eight and sixteen cell stages, etc., the position of this plane be 

 kept in mind, it will be found that the later blastomeres from 

 one or the other side often pass over the imaginary plane that 

 corresponds to the plane of first division. Striking, therefore, 

 as is the coincidence of the first plane of cleavage and the 

 middle plane of the embryo, it remains to be proved, I think, 

 that there is any direct causal connection between the first 

 cleavage-plane and the median line of the body. It may be 

 that the two phenomena are coincident because the internal 

 arrangement of the egg that determines one may also, but 

 independently, determine the other. In the newt Jordan ('93) 

 has shown that the first plane of cleavage corresponds approxi- 

 mately to the cross-plane of the body. That is, the first two 

 blastomeres correspond to the anterior and posterior parts of the 

 body respectively. He suggests that the shape of the egg-cap- 

 sule of the newt may be the cause determining the plane of first 

 division. Some other factor than that of the position of the 

 first plane of cleavage seems to determine the position of the 

 embryo on the egg, for in the teleost's egg, where the sym- 

 metry and bilaterality of the cleavage is even more sharply 

 marked than in the frog or newt, there seems to be no relation 

 at all between the first cleavage-planes and the planes of the 

 adult body. 3 



1 For method, see Pfliiger ('83), Roux ('85), and Morgan ('91). 



2 Rauber ('86) has later contradicted these results, but it is probable that 

 there is an error in his experiment. 



3 Clapp ('91), Morgan ('93). 



