310 



Embryogenesis: Progressive Differentiation 



region gives the highest proportion of dif- 

 ferentiation of each tissue. It is possible that 

 the thick mesodermal streak in this area 

 contains definitely localized autonomous pri- 

 mordia of notochord and somites, as well as 

 still migrating heart and nephros fields; 

 the results could equally well be interpreted 

 on the basis of high concentration of growth 

 factors plus a few key inducing substances, 

 quite irrespective of morphological localiza- 

 tions. 



Entodermal derivatives must largely be 

 considered in association with mesodermal 

 ones. Grafts from trout blastodiscs show a 

 distribution not very different from that of 

 somites, for example, for gut tubules (Fig. 

 1 19 A-C). In the chick, the progressive pat- 



Fig. 118. Progressive localization in the chick of 

 heart muscle and nephros (Butler, '35; Rudnick, 

 '32, '38b,c; Rawles, '43). The situation in the un- 

 incubated blastoderm is somewhat uncertain; it is 

 possible that some localization exists even then. It 

 is established that as soon as the piimitive streak 

 appears (5,C), a limited area only can form cardiac 

 muscle in vitro or in grafts (D,E), and that imtil the 

 head-process stage the median part of this area is 

 most active. In E and G the boundaries of the areas 

 have been carefully determined; in the other stages, 

 they are somewhat conjectural, the shaded areas 

 representing positive cases. 



structiu-es: heart and nephros. Heart-forming 

 areas (i.e., those which will form cardiac 

 muscle) follow fairly well the course that 

 would be expected of invaginating lateral 

 mesoderm, as probably does the more pos- 

 terior nephric field. A comparison of the 

 various diagrams for the definitive primitive 

 streak stage (Figs. lUE, 118D, F) shows a 

 remarkable concentration of potencies in 

 the immediate vicinity of Hensen's node: 

 an expression of the experimental fact that 

 grafts from this center develop into larger, 

 more varied masses than do more lateral or 

 anterior or posterior regions; the medianmost 



Fig. 119. Localization of entodermal potencies: 

 A-C from Luther ('36a); D-F from Butler ('35), 

 Rudnick and Rawles ('37); G~J from Rudnick ('32) 

 and Rawles ('36); data all from grafts. In the chick 

 material, lateral and posterior limits of areas have 

 not been determined in stages earlier than the head 

 process. In D-F, the shading indicates gut not 

 formed into recognizable organ units; crosses, or- 

 ganized small intestine; circles, identifiable seg- 

 ments of colon. 



