172 



INTERNAL FACTORS 



IV. i 



mere the yolk sinks next to the plane of separation, while the 

 protoplasm and pigment rises to the outer side. The two cells are 

 now related as two whole eggs united by their vegetative poles, 

 their axes in one and the same (horizontal) straight line; each 

 has, in fact, acquired a totally new polarity of its own. When 

 segmentation has been completed a groove appears, in the plane 

 of separation, and gradually extends round the whole circumfer- 

 ence; the ends of the groove are forked, but the branches of 

 each fork unite as the groove grows round. The groove is, in 

 fact, a blastoporic lip common to the two, the branches the 

 individual lips, the material between them and finally covered 

 over by them a common yolk-plug, the space between the two 



Fio. 86. Section through a 

 double blastula of the Frog 

 (Rana fusca). k, blastocoels. 

 FIG. 85. Double embryo ob- (After Wetzel, from Korschelt 



tained by the same method : h, and Heider.) 



heads ; m, medullary grooves ; 

 c, line of union of the latter. 

 (After Wetzel, from Korschelt 

 and Heider.) 



a common archenteron extending into an archenteric space in 

 each individual. Subsequently medullary folds and notochord 

 are developed in each below the groove, that is on the vegetative 

 or postero-dorsal side of each, but anteriorly each grows out free 

 of the other, and in this region medulla, notochord, and gut are 

 single. The result is, therefore, two embryos placed back to 

 back, and united by a common yolk-plug. 



Experiments in which the four animal or the four vegetative 

 cells of the eight-celled stage are killed are not very conclusive, 

 as the development of the survivors does not go veiy far. 



