248 



Embryogenesis: Progressive Differentiation 



pective significance, but when grafted to the 

 somite and pronephros level, they differenti- 

 ate "neighborwise," probably as a result of 

 assimilative induction (Yamada, '37, '40). 

 However, when ventral mesoderm was iso- 

 lated in combination with notochord, it fre- 

 qviently gave rise to pronephric tubules and 

 the blood cell formation was suppressed 

 (Yamada, '40; Fernald, '43; Muchmore, '51; 

 see Fig. 86). 



This situation has been interpreted in 

 different ways: Modifying the ideas of Dalcq 

 and Pasteels ('37, '38), Yamada ('40) as- 

 sumes that the qualitative differences in the 

 differentiation of the mesoderm originate 

 from quantitative differentials of the morpho- 

 genetic activity within the mesoderm mantle. 

 Each specific differentiation would be the 

 manifestation of a certain "morphogenetic 

 potential," and (arbitrarily assigned) thres- 



Prospecfive 

 significance 



In 

 explants 





Cultured 

 wi + h notochord 



Fig. 86. Schematic representation of the differentiations of prospective somite, pronephros, and blood 

 island regions, respectively, of the early neurula. First vertical row: normal fate; second vertical row: 

 differentiation when isolated in ectodermal vesicle; third vertical row: differentiation when combined with 

 notochord, in ectodermal vesicle. (After Yamada, '40.) 



All these data show that some parts of the 

 mesoderm of the early neurula are endowed 

 with stronger differentiation tendencies to- 

 wards their prospective fate than they were 

 in the early gastrula stage. This applies es- 

 pecially to the prospective notochord. The 

 pronephric and ventrolateral areas, while 

 capable of differentiating selfwise and in- 

 dependently of each other, can still be shifted 

 into other routes of differentiation when ex- 

 posed to the influence of prospective noto- 

 chord or somites. The prospective somite 

 region is particularly dependent on environ- 

 mental factors. Although its tendency to form 

 notochord or neural tissue is clearly reduced 

 as compared to the behavior of explants or 

 transplants from the blastoporal lip of the 

 early gastrula, its transformation into muscle 

 tissue is definitely enhanced by neighboring 

 tissues, among which the notochord plays a 

 predominant role. 



hold values would determine the lines of 

 segregation of the primordia. The "potential" 

 of the mid-dorsal area is presumably high 

 enough for notochord differentiation, but 

 that of the prospective somite area is too low 

 for muscle differentiation and just high 

 enough for the differentiation of pronephric 

 tubules. Combination with notochord results 

 in a raise of each level by one step: prospec- 

 tive somite and pronephros are raised to 

 muscle, lateral and ventral mesoderm to 

 pronephros differentiation. 



Muchmore ('51) envisages the organiza- 

 tion of the mesoderm mantle in terms of 

 several overlapping morphogenetic fields 

 which are characterized by qualitative rather 

 than quantitative differences. Tissues other 

 than the notochord can also direct meso- 

 dermal primordia into new channels of 

 differentiation, and there is no evidence that 

 these effects are due to quantitative grada- 



