DEVELOPMENT OF SPELERPES BILINEATUS 219 



fact that two of the blastomeres of the four-cell stage are ventral 

 instead of posterior, explains why no one, since Roux first described 

 a possible posterior half embryo, has succeeded in producing such 

 forms. Various explanations of this lack have been offered. 

 Morgan concludes that the "failure of the posterior lip to differ- 

 entiate further can only be accounted for as due to the absence 

 of the anterior parts." This opinion is'at variance with the well- 

 known capacity for self-differentiation of the various parts of 

 the egg. Brachet ('04), reached conclusions similar to my own, 

 although he thinks the absence of the anterior parts has some 

 effect in preventing the appearance of the blastopore. It would 

 be interesting to know whether or not a whole embryo, if any, 

 •would develop from the two ventral blastomeres if the egg were 

 inverted after killing the two dorsal blastomeres. 



From all of these considerations, I conclude that the amphib- 

 ian egg is not isotropic but is a mosaic. This mosaic does not 

 reveal itself in its mode of cleavage as happens in many other eggs. 

 The observed power of a single blastomere to produce a whole 

 embryo is dependent, not upon the totipotency of the blastomere, 

 but because it happens to contain the anlagen of all the parts of 

 the egg, which given opportunity, regenerates — to use a rather 

 doubtful term — the missing parts. 



Position of the embryo 



In 1905 and 1906, records by means of drawings were kept on 

 41 eggs, marked near the upper pole in early cleavage stages. 

 Sixteen of these gave definite results. The rest either died or the 

 stain faded or spread in such a way that no definite conclusions 

 could be drawn as to the position of the material for the anterior 

 connective in the uncleaved egg, although many showed that the 

 embryo must have developed largely upon the upper hemisphere 

 of the egg. None of them offered evidence that the embryo 

 developed mainly over the lower hemisphere. The eggs marked 

 to determine the relation between the first planes of cleavage 

 and the median plane of the embryo, confirm these results. With 

 two exceptions, which will be commented upon later, the same is 



