374 Florence Peebles. 



region caudad is evident. The injured area (w) is surrounded 

 by thickened ridges and back of the hole made by the wound the 

 notochord is present. Behind the notochord Hes the posterior end 

 of the primitive streak. No mesoblastic somites are present. 



The object of these experiments vv^as to prevent development 

 along the radius x-y by killing the cells in the region w^here the 

 primitive streak develops. In this w^ay it was hoped that the 

 primitive streak might be formed in some other part of the area 

 pellucida. The results show very clearly that no other part of the 

 blastoderm is capable of forming the primitive streak. They also 

 show that the region of the unincubated blastoderm along the 

 radius x-y is the region from which the mesoblastic somites 

 develop, /. ^., the trunk region of the embryo. From these ex- 

 periments it seems evident that the position of the embryo upon 

 the blastoderm is determined before the egg has been incubated, 

 and probably before segmentation is completed, for some of the 

 eggs which I used were operated upon within two hours from the 

 time that they were laid. 



II. THE DIRECTION OF GROWTH OF THE EMBRYO BEFORE AND 

 AFTER THE FORMATION OF THE PRIMITIVE STREAK. 



MarshalF describes the growth of the blastoderm from the be- 

 ginning of incubation as follows: "After incubation has com- 

 menced, the blastoderm spreads rapidly, retaining its circular 

 shape. By the end of the first day of incubation it is about the 

 size of a sixpence, and by the end of the second day it has 

 extended nearly halfway round the egg." 



According to DuvaP the edge of the blastoderm advances over 

 the egg at every point except at the posterior margin, and the 

 edges on each side of this point meet each other in the middle line 

 to form the primitive streak ("plaque axiale"). Assheton's^ 

 experiments have proved, however, that the growth is symmetrical 

 as Marshall states. 



'Marshall. Vertebrate Embryology. 

 ^Duval. Loc. cit. 

 ^Assheton. Loc. cit. 



