374 



Special Vertebrate Organogenesis 



the observation that isolated sections of the 

 spinal cord, transplanted to the flank, where 

 they are deprived of axial stretch, do become 

 much thicker and shorter than if they had 

 developed in continuity with the rest of the 

 cord (Zacharias, '38). The ease with which 

 lateral halves of the cord are regenerated 

 with the participation of transverse cell 

 shifts from the intact half (Detwiler, '47b; 

 Harrison, '47; Holtzer, '51), contrasted with 

 the failure to repair major gaps in the longi- 

 tudinal direction, likewise indicates the 



tivity declines at some levels and flares up at 

 others. Moreover, the centers of proliferation 

 often form a quiltlike pattern with foci at the 

 intersections of four longitudinal columns 

 with transverse bands (neuromeres) (Berg- 

 quist and Kallen, '54). Some of the segmental 

 peaks coincide with the appearance of promi- 

 nent peripheral organs in the corresponding 

 sector, for instance, in the limb segments. 

 As will be shown later (p. 381), part of this 

 correspondence is due to an active control ex- 

 erted upon the centers by their respective 



5 lO IS 



Fig. 140. Mitotic pattern of the brain of Amblystoma (after Burr, '32). A, Earlier stage (31, after Harri- 

 son). B, Later stage. (35, after Harrison). 



To facilitate comparison, the two stages are represented side by side in symmetrical arrangement. The 

 outline of the brain is indicated. Horizontal lines express the number of mitotic figures contained in serial 

 brain slices of 30/i each. Abscissa: Number of mitoses. Full black: Mitoses in forebrain and hypothalamus. 

 Individual bars: Mitoses in the rest of the brain. Brackets indicate position and extent of sensory placodes. 

 Note the change in the distribution of peaks from A to B. 



need of extraneous stretch for elongation. In 

 the brain, the hydrostatic turgor deserves 

 similar attention as a potential regulator of 

 the expanse of the proliferative surface, 

 hence of the over-all growth rate. These con- 

 siderations would not apply, of course, to 

 those parts of the CNS which arise, not as 

 transformations of the original neural plate, 

 but by secondary budding processes (e.g., 

 the posterior parts of midbrain and spinal 

 cord in amphibians). 



Regional Patterns of Proliferation. Regional 

 variations of mitotic activity within the 

 germinal layer have been mapped out for 

 several species and stages (urodele spinal 

 cord: Coghill, '24, '36; urodele brain: Burr, 

 '32; chick cord: Hamburger, '48; comparative 

 studies: Bergquist and Kallen, 53b). Each 

 stage has its own characteristic pattern (Fig. 

 140); that is, as development proceeds, ac- 



peripheries. But there are also some basic 

 intrinsic growth differentials among differ- 

 ent regions of the CNS (Coghill, '36; Det- 

 wiler, '24a, '36b; Hamburger, '48), the origin 

 of which remains in need of explanation. 



It has been proposed that local prolifera- 

 tion is regulated by the number of intra- 

 central tract fibers ending in a given locality 

 (Detwiler, '36b). This has been inferred 

 from experiments in which posterior cord 

 pieces of potentially smaller end size (of 

 urodele tail-bud stages) and anterior ones 

 of potentially larger size had been mutually 

 exchanged and found to develop in accord- 

 ance with their new sites, the former growing 

 beyond, the latter remaining below, the sizes 

 they would have attained in their original 

 positions (Detwiler, '23, '24a). Since more 

 descending brain fibers terminate at more 

 anterior than at more posterior levels, these 



