EMBRYONIC RECONSTITUTIONS 529 



median parts — with reconstitution of more lateral regions. This author 

 also holds that his data support Dalcq's view that the inductor differ- 

 ences along the polar axis are quantitative, not quahtative. 



Some of the earlier experiments on chorio-allantoic grafting were more 

 directly concerned with questions of the degree of organization or deter- 

 mination attained at particular stages than with spatial pattern and led 

 to the conclusion that progressive organization is arrested in a part by 

 isolation; consequently, the differentiation of the isolated part is a measure 

 of the degree of organization present at the time of isolation (Hoadley, 

 1926(3, 6; 1927). If this is true, it follows that early isolation of parts 

 should give less differentiation than isolation at later stages and that re- 

 constitution should not take place, but other experiments show extensive 

 reconstitution. 



Transplantations of parts of the unincubated blastoderm show exten- 

 sive differentiation only when they include the intact posterior median 

 quadrant, the region from which the primitive streak later develops. This 

 quadrant alone, although much smaller than the anterior three-fourths, 

 gives rise to almost all the tissues of the anterior embryonic region and 

 shows as extensive differentiation as posterior halves or whole blastoderms 

 or the node-level of whole blastoderms of definitive primitive-streak stage 

 (Butler, 1935). This posterior quadrant is the most susceptible region of 

 the early blastoderm (p. 162). According to Butler, pieces lacking this 

 posterior region do not develop central nervous tissue and usually give 

 rise only to gut, smooth muscle, heart, liver, and skin tissues. Pieces in- 

 cluding part of this posterior region show higher frequency of graft devel- 

 opment than anterior pieces, and transverse fourths from different levels 

 show a posteroanterior gradient of decreasing frequency of development. 

 Fourths from the posterior half show much less development than the 

 whole half or the median quadrant; and longitudinal fourths show little 

 differentiation, giving only gut, smooth muscle, and heart, probably, as 

 Butler suggests, because of interference with the cell movements con- 

 cerned in development of the primitive streak. 



In very early streak stages anterior, purely ectodermal regions, ex- 

 planted to plasma clots, may form neural tubes; pieces anterolateral to the 

 streak form chiefly heart in stages before the groove appears, after that, 

 heart and neural plate ; and the region of the streak gives rise in pregroove 

 stages only to erythroblasts, in groove stages to heart and erythroblasts 

 (Rudnick, 1938&). These results differ widely from those obtained by But- 

 ler with chorio-allantoic grafting of pieces from unincubated blastoderms. 



