242 



Embryogenesis: Progressive Differentiation 



becomes converted into head tissues (Ton- 

 dury, '37; Shen, '37; Holtfreter, '38b; Dalcq, 

 '40). A similar convertibility has been ob- 

 served when the dorsal blastoporal region 

 comprising the presumptive head and trvmk 

 mesoderm is rotated 180 degrees (Wadding- 

 ton and Yao, '50). Musculature forms readily 

 from prospective notochord, as is shown in 

 experiments of Spemann and Bautzmann 

 ('27), Weber ('28), Wang ('33) and Mayer 

 ('35). The rotation and exchange experi- 

 ments within the marginal zone performed 

 by Bautzmann ('33) demonstrate, in agree- 

 ment with numerous other observations, that 

 prospective lateroventral mesoderm can be 

 transformed into somites and notochord. 



As was found in explantations, the chorda- 

 mesoderm material is furthermore capable 

 of forming not only mesodermal bvit also 

 various epidermal and neural structures. 

 This occurred, for instance, when a piece of 

 the blastoporal lip was grafted into another 

 embryo, or into a jacket of ectoderm (Lewis, 

 '07; Spemann, '18; Spemann and Mangold, 

 '24; Holtfreter, '36; Dalcq and Lallier, '48a, 

 b). Following excision of a large portion of 

 the ectodermal roof, the defect was closed by 

 the expanding marginal zone and a normal 

 animal could be formed; part of the prospec- 

 tive chorda-mesoderm produced, according 

 to its new position, various ectodermal struc- 

 tures (Bruns, '31; Holtfreter, '38b). When a 

 large dorsal piece including the borderline 

 areas of prospective medullary plate and 

 mesoderm was rotated 180 degrees the dis- 

 located areas developed according to their 

 new position, demonstrating once more their 

 convertibility into each other (Tondury, '36) . 

 However, when the prospective head meso- 

 derm of an early gastrula was replaced by 

 prospective trunk-tail mesoderm of an ad- 

 vanced gastrula, no regulation occurred: the 

 embryo developed merely a spinal cord but 

 no head structures (Hall, '37). Obviously, 

 apart from a chorda-mesodermal trunk field, 

 the early gastrvila possesses a mesodermal 

 head field, and these two cannot be readily 

 converted into each other. 



It is unfortunate that the factors which 

 determine the formation of tissue patterns 

 within the marginal zone have so far eluded 

 further analysis. We shall face the same issue 

 in the discussion of the neurula stage, during 

 which the major segregations take place. The 

 general result of all these studies is epito- 

 mized in the statement that the chorda- 

 mesoderm area of the gastrula represents an 

 equipotential field with diffuse outlines and 

 a center of activity near the dorsal lip; this 



is little more than a formulation of the 

 problem. However, another important idea 

 emerges from these experiments: Whereas 

 the chorda-mesoderm field shows a high 

 degree of self-organizing capacity when ex- 

 planted, its actual fate in the embryo depends 

 not only on determinative interactions with- 

 in this area but also upon its topographic 

 relations to the ectodermal (and entodermal) 

 parts of the embryo. Apparently, the sur- 

 rounding ectoderm is involved in fixing the 

 boundaries of the material which is destined 

 to invaginate and to become mesodermal. 

 This relationship is quite remarkable since 

 later on it is the invaginated material which 

 determines the blocking out and differentia- 

 tion of the ectodermal structures. 



EXPERIMENTS WITH CHEMICAL AND 

 PHYSICAL AGENTS 



Further evidence of the labile nature of 

 determination of the entire marginal zone 

 comes from experiments with externally ap- 

 plied chemical or physical agents. Exposure 

 of early Tritvirus gastrulae to certain con- 

 centrations of lithium chloride resulted in 

 larvae lacking all or parts of the notochord 

 (F. E. Lehmann, '33, '37, '38; Pasteels, '45). 

 Vital staining experiments of Lehmann have 

 shown that the prospective notochord was 

 used in the formation of somites. When ex- 

 plants of prospective lateroventral mesoderm 

 were briefly svibjected to the disaggregating 

 action of ammonia, they differentiated partly 

 into notochord and somites (Yamada, '50a), 

 and when whole frog gastrulae were exposed 

 to urea solutions, scattered fragments of 

 notochord appeared within such different 

 tissues as the somites, the neural plate, and 

 the entoderm (Jenkinson, '06; Fautrez, '49). 

 Treatment of frog blastvilae with thiocyanate 

 resulted in an increase of the notochord di- 

 ameter in the trunk region (Ranzi and 

 Tamini, '39; Ranzi, Tamini and Offer, '45- 

 '46). But since the outgrowth of the tail of 

 these larvae was delayed or subnormal and 

 since thiocyanate failed to convert explanted 

 lateroventral mesoderm into notochord 

 (Ranzi and Tamini, '40), it is likely that the 

 increase in diameter resulted simply from 

 a failure of the notochord to elongate, an 

 effect which need not involve any tissue con- 

 version since it can be likewise achieved by 

 mechanical inhibition of the tail formation 

 (Holtfreter, '45a). 



Suppression of notochord formation has 

 been observed, furthermore, in amphibian 

 larvae which at early stages had been sub- 



