Nervous System 



371 



occurs even in asymmetrically mutilated 

 embryos under mechanically wholly aber- 

 rant conditions (Holtzer, '51), it cannot be 

 simply a mechanical accident, but must be 

 viewed as a case of selective fusion of tis- 

 sues according to their respective affinities 

 (see Holtfreter, '39; Chiakvilas, '52). Simi- 

 larly, the extrusion at this stage of the 

 neural crest cells from the confines of the 

 neural plate might be an expression of a 



cylindrical lumen. This actually occurs in 

 isolated pieces of plate in homogeneous sur- 

 roundings (Holtfreter, '34). The slit-shape 

 of the normal tube has been shown to de- 

 pend on the presence of notochord (Leh- 

 mann, '35) (Fig. 138C,jE). The effect may 

 be credited to a vertical system of fibers, 

 spanning the thickness of the plate along 

 a median strip coextensive with the noto- 

 chord and apparently attached to it, which 



^s 



Fig. 138. Shape of neural tube under different conditions (from Holtfreter, '34). A, Solid neural mass 

 developed in explantation: nuclei of gray matter crowded near the surface; white matter in the interior. 

 B, Neural tube surrounded by mesenchyme: shape cylindrical with central lumen; nuclei massed at the 

 inner (free) surface. C, As5Titaxia dorsalis (failure of the tube to close); thinning of the floor of the tube 

 in contact with the notochord; gray matter along the free surface. D, Neural tube underlain by muscula- 

 ture; lumen eccentric at far side; white matter at near side. E, Neural tube underlain by notochord; 

 normal appearance; slit- shaped lumen, oriented towards notochord. /, Limien; m, mesenchyme; c, noto- 

 chord; s, segmented musculature. 



transient incompatibility (or disaffinity) be- 

 tween the two groups. Thus, visible move- 

 ments and shaping processes appear as the 

 observable results of more intimate physico- 

 chemical distinctions within the prospective 

 neural system. 



Early Morphogenesis of Brain and Cord. 

 The gross shapes of the early brain and 

 cord, respectively, are anticipated in the 

 proportions of the neural plate, whose wide 

 anterior part, upon folding upward, forms 

 the large vault of a brain ventricle, while 

 the narrower posterior part encloses the 

 narrow lumen of the central canal of the 

 spinal cord. 



The shape of the canal varies with the 

 details of the folding process (Fig. 138). Uni- 

 form curling of the plate would leave a 



holds the midline firmly anchored as a 

 hinge about which the flanks of the plate 

 fold up (Weiss, '50c). A similar fibrous 

 plane seems to define the border between 

 the alar and basal plate cell masses; as the 

 latter grow and bulge, it gives rise to the 

 lateral sulcus. Because of their importance 

 for the later regular distribution and group- 

 ing of cell cokmins, svich tangible traces of 

 early subdivisions would merit more in- 

 tensive study; at present, we have no more 

 than vague hints as to their presumable 

 role. 



After the closure of the groove, the turgor 

 of the fluid in the lumen assumes the mor- 

 phogenetic role of firm support for the limp 

 walls, which otherwise would collapse. The 

 source of this turgor has been found in 



