174 



INTERNAL FACTORS 



IV. i 



The discovery of Herlitzka has been followed up by the 

 researches of Spemann, who has employed the same method of 

 constriction, but with variations of degree, direction, and time. 

 These differences have given the most interesting results. 



The first furrow, according to this author, is usually at right 

 angles to the sagittal plane and separates the material for the 

 dorsal and ventral halves of the embryo; only rarely do sagittal 

 plane and first furrow coincide. Both cases were, however, 

 experimentally investigated. 



Fia. 88. Three stages in the production of a double monster by 

 strong median constriction of the Newt's egg. (After Spemann, 1903.) 

 a. Beginning of gastrulation ; there is a separate lip in each half. 

 6. 7. and r. Med., Medullary folds of left and right embryos ; *, point 

 where the medullary grooves separate ; Bl, blastopore. c. The double- 

 headed larva. 



When the first furrow is in a horizontal plane, a slight con- 

 striction in the two-celled stage separates the dorsal lip of the 

 blastopore in one portion from the ventral lip in the other. 

 Medullary folds are developed in the first half only, the second 

 forms a sort of yolk-sac appendage which is later absorbed by 

 the single normal embryo. With tighter, but still incomplete, 

 constriction the dorsal half alone becomes an embryo ; it contains 

 either the whole or only the dorsal portion of the blastopore. 

 The ventral half contains either no portion of the blastopore or 



