92 



THE CELL AND PROTOPLASM 



thyreoglobulin and thyroxin, have a marked 

 effect on the plumage of birds. 



The feather is a complicated structure 

 and no complete understanding of the effect 

 of the internal medium upon its differenti- 

 ation is possible without detailed knowledge 

 of its development and growth. For the 

 present purpose, however, it will suffice to 

 say that the epithelial feather germ grows 

 from its base, the tip of the feather being 

 pushed out from within (Lillie and Juhn 

 1932). The more distal barbs are therefore 

 the older. The individual barbs grow like- 

 wise from the base, and their growth rate 

 is higher at first, i.e., when the tips of the 

 barbs are formed, and it gradually tapers 

 off. The barbs are attached obliquely to 

 the shaft so that a transverse bar on a 

 feather cuts successive barbs at different 

 levels. When a feather is plucked, it re- 

 generates from the remaining follicle in es- 

 sentially the same way as the original 

 feather develops. Thus it is possible to 

 carry out on adult birds experiments which 

 show the effect on the feather pattern of 

 substances circulating in the internal 

 medium. 



The response to injections of thyroxin 

 is expressed by a dark pigment spot or 

 stripe on the brown or yellow feathers of 

 the brown Leghorn chicken. The same in- 

 jection, if the amount is appropriately 

 gauged, may affect individual feathers dif- 

 ferently according to their position on the 

 body of the bird. This is owing to certain 

 factors, which vary according to position. 

 The threshold of reaction and the rate or 

 amount of reaction are directly propor- 

 tional to the growth rate, and the latent 

 period is inversely proportional (Lillie 

 1932). Since these factors are different in 

 the individual barbs, the effect produced on 

 the feathers may vary all the way from a 

 narrow spindle-shaped black spot centered 

 on the shaft to a transverse bar, running 

 clear across the feather. As many as five 

 or six successive injections, if properly 

 spaced, may be recorded in this manner on 

 a single feather. The female sex hormone 

 has similar effects when injected into 

 capons. According to dosage and position 



it produces brown spots or bars on feathers 

 that without treatment would have become 

 black, or it may produce whole brown 

 feathers. These hormones also affect the 

 form and texture of the feathers. 



The natural barring of feathers, such as 

 occurs in the Plymouth Rock breed, is ap- 

 parently brought about in a different way 

 (Montalenti 1934). It is to be referred 

 rather to a rhythmic activity of the genes 

 in each feather follicle, the exact nature of 

 which is unknown. The fact that the same 

 phenotypic effect may be produced in such 

 different ways is of great interest. 



Conclusion 



The foregoing sketch, which makes no 

 pretense to completeness, calls attention to 

 some of the facts regarding the action of 

 the internal environment upon the differ- 

 entiation of cells, i.e., upon the process of 

 dependent differentiation in contradistinc- 

 tion to self -differentiation (Roux 1885). 

 With the growing mass of evidence for the 

 former and, hence, for the theory of epi- 

 genesis, an exhaustive treatment of the sub- 

 ject would extend far beyond the scope of 

 a single paper. So, bearing in mind the 

 examples given above, we may now review 

 the salient points in the theory of develop- 

 ment here presented. 



The egg cell from which each organism 

 is derived is composed of protoplasm pecu- 

 liar to the species and different from that 

 of any other species. This substance is 

 laid down with collaboration of the nucleus 

 and in continuity with the tissues of the 

 maternal parent, perpetuating its specific 

 characteristics thereby. The growth period 

 of the ovum is thus an important one in 

 the development of the individual — one 

 that has been relatively neglected by the 

 embryologist. 



The material is deposited in the egg ac- 

 cording to a certain configuration which is 

 oriented in relation to the surrounding pa- 

 rental tissues. This configuration must be 

 conceived as related to the molecular struc- 

 ture of the protoplasm, more particularly 

 to that of the proteins. Although the gross 

 pattern of the egg is much changed at the 



