L 



DEVELOPMENT OF SPELERPES BILINEATUS 237 



should expect that the distal end would remain stationary and 

 the proximal end would finally be found at the posterior end of 

 the embryo, the mark meanwhile having extended into a band. 

 A series of marks made about the equator before gastrulation 

 would, at the close of gastrulation, be found in bands marking 

 out meridians on the lower surface of the egg.^* If there were 

 any concrescence, one would expect that the exovates made by 

 Miss King in the dorsal lateral lip would have moved into the 

 same meridian as those made at the middle of the dorsal lip. 

 Having reached the median line, they should then have remained 

 stationary, if Morgan's view of the formation of the embryos 

 were correct. They not only do not move into the median line, 

 but an exovate made in the middle of the dorsal lip of the blasto- 

 pore, where the material is supposed to be coming up from the 

 lateral lips and fusing, actuall}' moves down to the posterior end of 

 the embryo. The actual movements of the exovates completely 

 substantiate the view of the formation of the posterior part of 

 the embryo by convergence. 



The view of embryo formation by concrescence agrees with the 

 view of formation by convergence in one essential point, viz.: 

 The material out of which the embryo, or at least its posterior 

 part, is to form lies in a half ring about the equator. The middle 

 of this half ring corresponds to some point in the median line of 

 the embryo. Here the two views part company. According 

 to the former the lateral halves of the embryo lie along this region 

 and join by a movement transverse to the long axis of the embryo. 

 According to convergence, the material out of which the embryo 

 is to form, lies in this same region, but comes together along lines 

 oblique to the long axis of the embrj^o. But more fundamental 

 yet are the differences between the two views regarding the con- 

 stitution of the material of the equatorial ring. According to the 

 theor}^ of concrescence — in the frog, to take a specific instance — 

 the material for the head lies just above the dorsal lip. As we 

 pass back along the ring, we come successively to material for 



^^ More recent experiments on the eggs of the frog and of Amblystoma confirm 

 these statements. 



THE AMERICAN JOURNAL OP ANATOMY, VOL. 12, NO. 2 



