Nervous System 



375 



fibers were thought to have determined the 

 site-specific growth rates. Confirmatory evi- 

 dence was seen in the fact that when the 

 anterior cord segments were replaced by a 

 supernumerary medulla as extra fiber source, 

 the spinal host segments lying immediately 

 posterior to the graft became abnormally 

 large (Detwiler, '25c). 



In the light of later work, however, this 

 view has become untenable. For instance, 

 when the normal medvilla as the supplier of 

 supposedly stimulating fiber tracts was re- 

 placed by a spinal fragment of much lower 

 fiber productivity, the host spinal cord be- 

 hind developed quite normally, without the 

 expected diminution (Detwiler, '37c). Simi- 

 larly, suppression of forebrain development 

 entails no deficienies in the medulla (Det- 

 wiler, '45). In the chick, cord segments which 

 have been transected and prevented from re- 

 ceiving down-growing fiber tracts still gain 

 normal dimensions (Levi-Montalcini, '45; 

 Hamburger, '46). Also, the assumption that 

 descending fibers would have an intrinsic 

 preassigned length at which they would 

 stop is gratuitous; free nerve fibers do not 

 stop spontaneously, but are stopped by their 

 surroundings, usually a recipient cell. Hence, 

 fiber tracts cannot be the major determinants 

 of axial growth patterns, although they can 

 exert some modifying effects (see p. 383). 



All evidence thus leads to the conclusion 

 that the closed neural tube represents a longi- 

 tudinal mosaic of specifically different local 

 fields, each guiding the further fate of the 

 area under its control; differential prolifera- 

 tion is but one expression of these different 

 fates, with which we shall deal more fully 

 below in the proper context (p. 376). 



Besides the longitudinal pattern, there is 

 also a notable dorsoventral differential of 

 mitotic activity, both in regard to intensity 

 and time course. Proliferation is more abun- 

 dant and lasts longer in the dorsal than in 

 the ventral half of the cord, with a rather 

 sharp demarcation between the two zones 

 (see Fig. 139). Generally noted in amphib- 

 ians (Detwiler, '25a; Maclean, '32; Coghill, 

 '33), this fact has been most conclusively 

 demonstrated in the chick (Hamburger, '48). 

 It may be related to the general precocity 

 of the ventral, as compared to the dorsal, 

 portions, including the precession of motor 

 over sensory function (Coghill, '29). Dorsal 

 and ventral halves also differ in other re- 

 spects, the ventral one being far richer in 

 alkaline phosphatase (Moog, '43), and at 

 the same time being the first to receive 

 vascularization (Feeney and Watterson, '46). 



Growth Rate. Conventionally, the term 

 "growth rate" refers to average increment 

 of an organic object per unit of time. It is 

 determined by measuring the object at the 

 beginning and end of given intervals and 

 dividing by the times elapsed. The resulting 

 values are useful for rough orientation, but 

 are often meaningless for analytical and 

 comparative purposes, except for homogene- 

 ous systems. If the dimensions of one part 

 of the CNS increase faster than those of 

 another part, this need not mean that the 

 intrinsic growth activity of the former has 

 been greater or that its mitotic rate has been 

 higher. If we consider, for example, a given 

 subdivision of the CNS bounded by certain 

 landmarks by which it can be identified at 

 successive stages, its volume increase is the 

 resultant of the following tributary proc- 

 esses: cell growth, accompanied by division 

 in the germinal layer; cell growth without 

 division in the mantle; immigration of nerve 

 cells and other cell types (glia) from neigh- 

 boring regions; emigration of cells; out- 

 growth of axons and deposition of myelin 

 (with that portion of the axons which mean- 

 while has moved beyond our landmarks 

 being unaccountably lost); passage of axons 

 from other areas; invasion of blood vessels; 

 accumulation of interstitial fluid; and cell 

 disintegration. Large-scale destruction and 

 resorption of cells is a common, if neglected, 

 feature of embryonic development (Gliicks- 

 mann, '51), and as we shall see presently, 

 is quite prominent in the embryonic nervous 

 system. 



From this listing, it should be plain that 

 comparisons of the "growth" of different 

 parts of the CNS on the basis of mere 

 measurements of volume or mass cannot be 

 very revealing and must be interpreted with 

 due caution. There is an even wider margin 

 for error when, instead of volume deter- 

 minations, only one, supposedly representa- 

 tive, dimension is sampled; e.g., cord diam- 

 eter. The case is illustrated by the fact that 

 the increase in area observed in cross sec- 

 tions of isolated cord segments (Severinghaus, 

 '30) often was interpreted as hyperplasia un- 

 til more complete determinations disclosed 

 that there had been a corresponding reduc- 

 tion in length (Zacharias, '38) (see p. 374). 



This comment should not detract from the 

 value of the summary quantitative treatment 

 usually accorded to CNS growth, which has 

 been a true advance over earlier, purely 

 verbal, descriptions; rather is it to stress the 

 need for going even further and identifying 

 precisely and quantitatively the various 



