Amphibians 



241 



conclusion that a cell which in response to 

 inherent or external stimuli has become 

 determined to differentiate in a certain direc- 

 tion, acquires simultaneously the tendencies 

 of motility and adhesion characteristic of this 

 cell type. Varying with the type, the cell 

 may exhibit the tendencies of elongating or 

 spreading, of invaginating, aggregating, dis- 

 persing, etc. Proper execvition of these tend- 

 encies is a prerequisite for the final arrange- 

 ment of the cells into typical tissues and 

 organs. To manifest the tendencies, environ- 

 mental conditions of the kind mentioned 

 above are required. But cytological differenti- 

 ation may proceed to some length even if the 

 external conditions inhibit the execution of 

 collective cell movements. It is incorrect to 

 think that preventing the marginal zone 

 from invagination causes it to become ecto- 

 derm, but material which is determined to 

 become mesodermal tends to move into the 

 interior of an embryo. In conclusion, it may 

 be stated that the marshaling of the cells into 

 tissue patterns is controlled by a variety of 

 environmental factors which differ funda- 

 mentally from those which initially deter- 

 mine the material and dynamic trend of 

 cellular differentiation. 



TRANSPLANTATION AND DEFECT 



EXPERIMENTS ON THE 



EARLY GASTRULA 



The method of grafting certain parts of 

 the embryo into different regions of another 

 embryo involves two consequences: their 

 separation from the determinative factors of 

 the original tissue environment and, possibly, 

 their exposure to new determinative factors 

 at the site of implantation. Varying with the 

 new site and with the state of determination 

 of the graft, the tissue may develop accord- 

 ing to its prospective significance or accord- 

 ing to the determinative stimuli emerging 

 from the new tissue environment. If the 

 various grafts are subjected to a sufficiently 

 wide range of different stimvili, one may 

 expect to gain an idea of the range of devel- 

 opmental potencies of the grafts at the time 

 of operation. 



Potentialities of the Ectoderm. Whereas the 

 isolated ectoderm of the early gastrula lacks 

 the capacity of differentiation into typical 

 structures, it possesses a very wide range of 

 reaction potencies ("competence") when ex- 

 posed to inductive stimuli. The interchange- 

 ability of prospective epidermis and prospec- 

 tive neural tissue was demonstrated in the 

 •^lassie transplantation and rotation experi- 



ments of Spemann ('18). Corresponding 

 xenoplastic experiments gave similar results 

 (Holtfreter, '35a,b, '36). Transplantation to 

 the ventral region which lacks inductive 

 capacity resulted in the formation of un- 

 specialized epidermis (Mangold, '29a; By- 

 tinski-Salz, '29). 



Numerous experiments have shown that 

 any part of the early gastrula ectoderm can 

 be induced to form any kind of neural or 

 epidermal structure. Furthermore, given the 

 proper inductive stimuli, gastrula ectoderm 

 can be transformed into any kind of meso- 

 dermal tissue. When grafted into the latero- 

 ventral marginal zone of another gastrula, it 

 may form pronephric tubules or lateral plate 

 (Mangold, '23), and when grafted into the 

 dorsal blastoporal lip it can differentiate into 

 notochord, somites and neural tissue (Spe- 

 mann and Geinitz, '27; Raven, '38). In con- 

 tact with the caudal portion of the archen- 

 teron roof it can be induced to form noto- 

 chord, somites, neural tube, pronephros, 

 meninges, mesonephros, mesenchyme and 

 limb tissues (Holtfreter, '33c, '36; Raven, 

 '35; Spofford, '48). It has been shown that 

 this wide range of differentiation potencies 

 is equally shared by the presumptive epi- 

 dermal and medullary areas (Holtfreter, '33b, 

 '33c, '44b). There is no evidence for the 

 contention of Goerttler ('27) and Barth ('41) 

 that the ectodermal layer, before being af- 

 fected by inductive stimuli, possesses an in- 

 herent "polarity," or that its different areas 

 are in any way predetermined according to 

 their prospective fate. Whether or not the 

 gastrula ectoderm is fully omnipotent is still 

 an open question. Claims to the effect that 

 it can be transformed into blood cells, liver, 

 and gut tissues (Mangold, '23; Holtfreter, 

 '33c; Raven, '35, '38) require further elucida- 

 tion. 



The mesodermal marginal zone is endowed 

 with about the same range of differentiation 

 capacities as the ectoderm. Unlike the latter, 

 however, it has a certain tendency to form 

 mesodermal rather than ectodermal tissues, 

 and its various potencies can become manifest 

 independently of external inductive stimuli 

 by way of self-organization. Yet, the fol- 

 lowing observations indicate that determi- 

 native factors of the environment cannot 

 be disregarded, if one attempts to analyze 

 the field properties of the chorda-mesoderm. 

 Large defects in the median part of the 

 blastoporal lip involving removal of most of 

 the prospective head mesoderm can be regu- 

 lated almost perfectly. Obviously the sur- 

 rounding marginal zone, by filling the gap, 



