90 



THE CELL AND PROTOPLASM 



the animal as a whole, develops out of 

 the heteroplastic combination. This effect 

 must be attributed to the mutual regulation 

 of growth and division rate of cells by their 

 associated cells constituting the internal 

 environment (Harrison 1929). 



Within the nervous system there is much 

 interdependence in respect to growth and 

 differentiation between functionally associ- 

 ated nerve centers and between center and 

 end organ. Both hypoplasia and hyper- 

 plasia of centers have been observed, ac- 

 cording as connecting tracts are diminished 

 or increased in volume (Detwiler 1920, 

 1936; May 1933; Hamburger 1934, 1939). 

 A large eye, for instance, sends out a cor- 

 respondingly large optic nerve which stim- 

 ulates the mid-brain of the opposite side to 

 hyperplasia (Harrison 1929; Twitty 1932). 

 There are also influences emanating from 

 nerve centers that affect growth and de- 

 velopment of other organs, but this subject 

 is still largely controversial. One of the 

 most striking effects of this kind is the 

 tendency to atrophy or stoppage of growth 

 in muscles after severance of the nerve 

 supply. However, the initial differenti- 

 ation of muscle is not dependent upon its 

 connection with the nervous system (Har- 

 rison 1904; Hamburger 1928). 



As stated above, there is almost no evi- 

 dence ihat regions which are far apart 

 affect one another during early stages of 

 development, but as soon as the circulation 

 and nervous connections develop, factors 

 influencing differentiation become effective 

 through these media. An exhaustive treat- 

 ment of this subject would include a large 

 part of the field of endocrinology, with all 

 of the complications involved in the origin 

 and control of secondary sex characters. 

 Since this would require space greatly in 

 excess of that available, I shall limit myself 

 here to a single topic, the development and 

 distribution of pigment. 



Pigmentation in the lower vertebrates, 

 as well as in certain invertebrate forms, is 

 due to cells, known as chromatophores, 

 which are scattered in the mesenchyme 

 or concentrated on membranes surrounding 

 other organs such as blood vessels, fascia, 



peritoneum and meninges. In amphibian 

 larvae many lie embedded between the epi- 

 dermal cells. The pigment of the early 

 embryonic stages is already developed in 

 the ovarian egg and is merely distributed, 

 mainly to the ectoderm cells, during seg- 

 mentation. 



The pigmentary system of vertebrates, 

 both in its developmental stages and later, 

 is subject to a variety of influences — inter- 

 cellular, nervous, humoral and outer en- 

 vironmental (temperature and light). 

 Moreover, the several actions may be medi- 

 ated in different ways, either directly or 

 reflexly. The processes involved are there- 

 fore complicated, and much ingenuity has 

 been exercised in working out experimen- 

 tally the interplay of factors. 



We are concerned here not with transi- 

 tory changes in pigment cells, which are 

 largely adaptive to external conditions, but 

 with quasi-permanent or irreversible trans- 

 formations involved in the development of 

 the pigmentary system. In the embryo the 

 prospective chromatophores are wandering 

 cells and hence their place of origin must 

 be ascertained before their distribution can 

 be understood. 



It has now been established beyond doubt 

 that the neural crest is the principal, if not 

 the sole, source of pigment cells (exclusive 

 of the retinal tapetum) in at least two 

 classes of vertebrates, amphibians (Du- 

 Shane 1935) and birds (Dorris 1939). The 

 neural crest is a strip of cells lying between 

 the neural plate and the ectoderm, which 

 becomes separated from both of these struc- 

 tures either during or shortly after the 

 closure of the neural folds. The pigment 

 cells thus start from a position on the dor- 

 sal side of the spinal cord and wander from 

 there ventrally under the ectoderm and in 

 the interstitial tissue. Associated with pre- 

 sumptive pigment cells in the neural crest 

 are other cells destined to give rise to a 

 variety of tissues — visceral cartilages, mes- 

 enchyme, spinal ganglion cells, sheath cells, 

 sympathetic ganglia, chromaffine tissue. 



This origin of pigment cells has been 

 demonstrated by means of tissue culture, 

 transplantation and defect experiments 



