Amphibians 



243 



jected to supranormal temperatures (Hoad- 

 ley, '38) or to centrifugation (Jenkinson, '14; 

 Pasteels, '40). When early amphibian em- 

 bryos were raised in a temperature gradient, 

 the warmed side developed faster than the 

 cooled one, but no other deviations from the 

 normal differentiation pattern were observed 

 (Huxley, ' 27; Vogt, '28b; Margen and 

 Schechtman, '39). Therefore, the notion of 

 Child ('29, '46) that simply a raising of the 

 physiological activity of a given embryonic 

 region can shift it into new trends of differen- 

 tiation does not apply to the amphibian em- 

 bryo. The few cases of accessory axial systems 

 wlaich Gilchrist ('28) observed after the local 

 application of higher temperatures were 

 probably due to a bifurcation of the in- 

 vaginating mesoderm rather than to a direct 

 determinative effect of heat upon the differen- 

 tiation of the ventral mesoderm or ectoderm. 



Irradiation of the blastoporal lip with 

 ultraviolet light tended to suppress its differ- 

 entiation and induction power (Dllrken, '36; 

 Reith, '38). When this treatment was applied 

 to the lateral half of the prospective chorda- 

 mesoderm the results varied with the stage of 

 the embryo (Brandes, '38, '40, '42). When 

 early cleavage stages of Rana were irradiated 

 unilaterally, the volumetric ratio between 

 notochord and somites became supranormal, 

 but when later stages, up to the early gas- 

 trula, were irradiated, it tended to become 

 subnormal. As compared with the somites of 

 the non-irradiated side of the blastoporal lip, 

 those of the irradiated side would attain, in 

 approximately equal percentages, a larger, 

 smaller, or eqvial volume, and no clear rela- 

 tionship could be established between the 

 dosage of irradiation and the tendency of 

 the marginal zone to form either somites 

 or notochord. 



Attempts to "explain" the experimentally 

 produced chorda-mesodermal conversions by 

 the hypothesis that a "morphogenetic po- 

 tential" (Dalcq and Pasteels, '37, '38; Pas- 

 teels, '40; Brandes, '42; Fautrez, '49) of the 

 affected primordia has been either raised or 

 lowered amount to little more than tautol- 

 ogy. There is no information available con- 

 cerning the chemical mechanisms involved 

 in these conversions. Nor do we know at 

 what critical point of this reaction chain 

 the various chemical and physical agents 

 have interfered, and why some of them 

 had a chordalizing, others a mesodermiz- 

 ing effect. The data sviggest that this con- 

 version proceeds with equal facility in either 

 direction. There is therefore no factual basis 

 for the assumption that the segregation of 



the chorda-mesoderm field results from the 

 differential distribution of a single hypo- 

 thetic svibstance, called "organisine" (Dalcq 

 and Pasteels, '38; Dalcq, '41a; Pasteels, '40; 

 Brandes, '42), and it is furthermore arbitrary 

 to assume that the prospective notochord 

 possesses a higher concentration of this sub- 

 stance, hence a higher "morphogenetic po- 

 tential" than the prospective somites. Some 

 less hypothetical suggestions have been made 

 as to the possible mechanism by which 

 lithium and other agents may bring about 

 their effects (reviewed by Gustafson, '50), 

 but it is clear that much more biochemical 

 work is needed before the conversions, and 

 the field characteristics of the marginal zone 

 in general, can be expressed in other than 

 morphogenetic terms. 



ANALYSIS OF THE "ORGANIZER" 



INDUCTIVE AND ORGANIZING 



ACTIVITY OF THE MATERIAL 



FROM THE UPPER 



BLASTOPORAL LIP 



In our discussion of the factors of organiza- 

 tion we have referred frequently to deter- 

 minative interactions between the germ 

 layers. The present section is devoted largely 

 to the inductive capacities of the chorda- 

 mesoderm field whose self-differentiating ca- 

 pacities have been dealt with in the preced- 

 ing section. 



In their pioneering work on the "organ- 

 izer," Spemann and H. Mangold ('24) trans- 

 planted part of the upper blastoporal lip into 

 the ventral ectoderm of another gastrula of a 

 different species. Another method of bring- 

 ing this material in contact with the ventral 

 ectoderm consists in inserting it into the 

 blastocoele. If the graft contained a sub- 

 stantial portion of the chorda-mesoderm 

 field, it invaginated, stretched itself between 

 ectoderm and entoderm of the host, tended 

 to establish the cavity of an archenteron 

 (Lehmann, '32) and then organized itself 

 into various tissues, such as notochord, so- 

 mites, pronephros, prechordal mesoderm and 

 entoderm (sometimes also neural tissue) 

 which we have already encountered in the 

 corresponding explants. Like the latter, the 

 graft tended to form a harmoniously pro- 

 portioned axial system with a craniocaudal 

 polarity and a bilateral symmetry. This 

 applied even to pieces containing only a 

 lateral half of the dorsal lip (Spemann, '31; 

 Holtfreter, '33d; Mayer, '35; Ekman, '36). 



Spemann ('21a) introduced the term "or- 

 ganizer" for the upper blastoporal lip. The 



