274 



Embryogenesis: Progressive Differentiation 



which becomes the ameboid anterior pole 

 when a cell migrates or exhibits "out- 

 growth." Inductive agents, whether repre- 

 sented by formed inductors or by injurious 

 aqueous media, act only when applied to 

 the uncoated cell surfaces or after the coat 

 has been dissolved (Holtfreter, '43a, '48b). 

 As a consequence of neural induction, 

 ectodermal cells acquire the tendencies (1) 

 to elongate reversibly into cylindrical bodies 



uptake, suggested by Glaser ('14, '16), do 

 not control the neurulation process (Brown, 

 Hamburger and Schmitt, '41). 



Determination of a cell to become a neu- 

 ron is immediately reflected in new tenden- 

 cies for adhesiveness and ameboid motility 

 (Holtfreter, '47a). An isolated Rohon-Beard 

 neuroblast adopts a pear shape, its anterior 

 pole projecting mobile filopodia into the liq- 

 uid medium and its tapering posterior pole 



Fig. 107. Kinetic tendencies of isolated embryonic cells, a, b, c, Ectodermal cells, determined to become 

 epidermis, exhibit flattening (epiboly) over the substrate; d, an isolated cell of the medullary plate elongates 

 into a cylindrical body; e, f, isolated neuroblasts form dendritic processes. (After Holtfreter, '48b.) 



(Fig. 107c?), (2) to move from the periphery 

 into the depth, (3) to detach themselves from 

 non-neural cells. The same three tendencies 

 appear in other induced ectodermal structures 

 (lens, otocyst, pituitary, placodes). By con- 

 trast, ectoderm cells which have not re- 

 ceived inductive stimuli and are destined to 

 form epidermis become spherical when float- 

 ing in a culture medium; they flatten out 

 when provided with a proper surface of 

 contact, and they do not invaginate (Holt- 

 freter, '47a, '48b; see Fig. 107a,b). These dif- 

 ferent kinetic tendencies are due to forces 

 inherent in each isolated cell and they are 

 the first cytological indication that induction 

 has occurred. According to the measurements 

 of Gillette ('44) these kinetic properties are 

 instrumental in normal development: the 

 medullary plate arises as the result of a 

 proximodistal elongation and corresponding 

 contraction of the outer surface of the dorsal 

 ectoderm cells, while the prospective epi- 

 dermis cells undergo a compensatory flatten- 

 ing. Volume changes by differential water 



having a pronounced adhesiveness (Fig. 

 107 e,f). Given an appropriate contact surface, 

 a neuroblast may migrate ameba-fashion, but 

 if the cell body remains attached, the ad- 

 vancing ameboid projections are spun out to 

 form branching processes, and the config- 

 uration of a dendritic neuron is obtained 

 (Harrison, '10). 



In a similar way, the morphogenesis of 

 other cell types, their properties of aggrega- 

 tion or dispersal, and the direction of their 

 movements, are largely determined by their 

 inherent axial organization. Stage-specific 

 and tissue-specific differences of motility and 

 adhesiveness are instrumental not only in 

 cyto-differentiation but also in organ forma- 

 tion. Holtfreter ('46a, '48b) has suggested 

 that shape and kinetic properties of embry- 

 onic cells result primarily from local and 

 temporal variations of the expansibility of 

 the cell membrane, reflecting changes in 

 the composition and molecular arrangement 

 of this structure which may be considered 

 as being essentially an organized lipo-protein 



