254 



Embryogenesis: Progressive Differentiation 



optic vesicle into a double-layered optic cup 

 (Holtfreter, '39c). 



The mesoderm adjacent to the spinal cord 

 plays apparently the same role in the bi- 

 lateralization of this structure as does the 

 head mesoderm in the bilateral distribution 

 of the eyes. If the prospective notochord is 

 extirpated (Lehmann, '26, '28; Horstadius, 

 '44) or its differentiation is suppressed by 

 exposure of early gastrulae to lithium chlo- 

 ride (Lehmann, '37; Pasteels, '45) or to cen- 

 trifugal force (Pasteels, '40), the somites may 

 fuse across the midline. Under these condi- 

 tions, the wall of the adjacent neural tube, 

 instead of forming the characteristic thin 

 floor plate, forms a thick mass (Fig. 88a). 

 This occurs likewise when an additional 

 neural tube is induced on the outer surface 

 of older somites (Holtfreter, '33c). On the 

 other hand, when neural tissue differentiates 

 combined with notochord alone, its contact- 

 ing side flattens out into a thin layer (Fig. 

 88c), while the presence of notochord to- 

 gether with imilaterally located somites pro- 

 duces the asymmetrical configuration of 

 Figure 88d. But when the spinal cord de- 

 velops surrounded merely by mesenchyme, 

 its walls are imiformly thick (Holtfreter, 

 '34c, '39c; Fig. 88e). Thus notochord and 

 somites not only represent the inductor sys- 

 tem for the spinal cord but they continue to 

 influence the cell distribution within the tube. 

 Although their formative effects are antago- 

 nistic, notochord and somites are normally 

 arranged in such a way that their effects 

 supplement each other, producing the typical 

 thinning of the floor plate and the thickening 

 of the lateral walls of the spinal cord (Fig. 

 88/). 



It should be mentioned that the presence 

 of an epidermal covering is equally import- 

 ant for the shaping of the neural tube, since 

 without it the medullary plate fails to close 

 and spina bifida (rachischisis) results. A 

 multitude of externally applied chemical or 

 physical agents can prevent the closure of 

 the plate into a tube. Many of them are 

 identical with the agents which inhibit the 

 movements of gastrulation (p. 237), e.g., 

 hypertonicity, extreme temperatiures, deter- 

 gents, iodo-acetamide. 



Undoubtedly, the archenteron roof has an 

 important share in the initial regional speci- 

 fication of the central nervous system. Yet 

 the experimental data give convincing evi- 

 dence that it is only partly responsible for 

 the subsequent morphological and histologi- 

 cal patterning of the morphogenetic fields 

 which it has projected onto the dorsal ecto- 



derm. At best, the inductive substratum 

 facilitates the subsequent processes in a 

 non-specific way. The elaboration of histo- 

 logical details is left to the self-organizing 

 capacity of the neural tissue which was re- 

 ferred to above. If adult tissue can induce the 

 ectoderm to form a typical bilaterally sym- 

 metrical brain flanked by any of the sense 

 organs (Fig. 99b), then it is plausible to 

 assume that similar autonomous processes oc- 

 cur within the different gradient fields of the 

 normal medullary plate. These self-organiz- 

 ing processes supplement the activities of the 

 archenteron roof and carry on when induc- 

 tions have ceased to operate. 



THE INDUCTION OF NEURAL CREST 



DERIVATIVES AND OF OTHER 



ECTODERMAL STRUCTURES 



We have followed the progressive differen- 

 tiation up to the neurula. In subsequent 

 stages, a great variety of new structures ap- 

 pears. Their morphogenesis is analyzed in 

 other chapters of this book. The present 

 chapter is limited to some general aspects of 

 the determination of ectodermal derivatives 

 not belonging to the central nervous system. 

 Prominent among them are the derivatives 

 of the neural crest which, in turn, originate 

 from the neural folds. We mention briefly the 

 following crest derivatives: the majority of 

 the chromatophores, large amounts of mesen- 

 chyme of the head and trunk region, the 

 principal components of the visceral carti- 

 laginous skeleton and of the cranial ganglia, 

 dental papillae, Schwann's sheath cells, at 

 least part of the corium and of the membran- 

 ous coverings of the brain, the chromaffin 

 bodies and the medulla of the adrenal glands 

 (see the reviews by Harrison, '38; Du Shane, 

 '43; Rawles, '48; Horstadius, '50). Ectodermal 

 placodes outside of the medullary plate pro- 

 duce the olfactory epithelium and its nerve 

 processes, the lens, the otocyst and its acous- 

 tic ganglion, the lateral line sense organs and 

 some components of the cranial ganglia. 



NEURAL CREST DERIVATIVES 



Since the neural crest originates in the 

 neural folds, one might expect that it is 

 induced in conjiuaction with the adjacent 

 medullary plate, by the more lateral parts 

 of the archenteron roof. Although some of its 

 derivatives, such as mesenchyme cells, chro- 

 matophores and Rohon-Beard neurons, can 

 be induced independently of medullary plate 

 tissues (Holtfreter, '33c; Raven, '35), these 



