EMBRYONIC RECONSTITUTIONS 535 



process, and early somite stages are grafted, parts posterior to the mes- 

 onephros do not develop in any case (Willier and Rawles, 193 1). Also, 

 grafted pieces from the more posterior levels of advanced primitive-streak 

 stages show, at best, only slight differentiation. The absence of posterior 

 parts in grafted whole blastoderms suggests that development is perhaps 

 unable to proceed beyond a certain stage on the chorio-allantois. Size of 

 piece may also play a part in determining occurrence and character of de- 

 velopment of grafted pieces, as is maintained by Murray and Selby (1930). 

 In postembryonic grafting in the lower invertebrates a larger piece very 

 generally persists and develops more frequently than a smaller. The small- 

 er fraction of gradient pattern in the smaller piece is less effective than a 

 larger fraction; but even small pieces from a high gradient-level, grafted 

 into a low level, may persist and develop. Similar questions arise with re- 

 gard to explantation. The explanted whole blastoderm may develop quite 

 normally up to a certain stage, at which development stops and death fol- 

 lows (WadcHngton, 1932). Interference with the cell migrations may also 

 be an important factor in hmiting realization of developmental capacities. 

 Explanted transverse strips of definitive primitive streak and head-process 

 blastoderms develop axial parts from the node-level and anterior regions, 

 but development of levels of the streak is interfered with by the trans- 

 verse sectioning (Rudnick, 1938a). The marked retardation of develop- 

 ment in explants of pieces of early blastoderms and the disappearance of 

 the streak structure from streak stages (Rudnick, 19386) suggests that de- 

 velopmental potencies of these pieces are far from realized in these experi- 

 ments. This may be due not to lack of organization but to the high sus- 

 ceptibihty of these stages to the conditions of explantation. According to 

 Butler (1935), the posterior quadrant of the unincubated blastoderm, 

 from which the primitive streak later develops, gives rise to axial organs 

 in chorio-allantoic graft; but in somewhat later stages the region of the 

 early streak in explants develops only erythroblasts (Rudnick, 19386). In 

 spite of the many positive results of experiment and the advance in knowl- 

 edge of avian development due to them, it still seems possible that devel- 

 opmental capacities of at least some parts of the blastoderm may be great- 

 er than experiment has shown. Some of the experimental data suggest 

 that the gradient pattern of later streak and more advanced stages de- 

 velops gradually in the blastoderm and that the apparent lack of organiza- 

 tion in early stages may be lack or inadequacy of this pattern, but they 

 throw no light on the problem of its origin. 



