56 University of California Publications in Zoology [VOL. 11 



As to the form and composition of these Mendelian factors, 

 Castle says we can at present give no satisfactory answer, but he 

 adds (p. 68) : "It is, however, we think, not necessary to suppose 

 that there exist in the minute germ-cell as many complex organic 

 substances as there are activities of the cell ; neither is it necessary 

 to suppose a different substance present for every independent 

 factor identified. The various independent factors may have a 

 basis no more complicated than that of so many atoms attached to 

 a complex molecular structure. Experiment shows that the factors 

 may be detached one by one from the organic complex. The 

 discontinuity of their coming and going is entirely in harmony 

 with the conception of them as components merely of complex 

 molecular bodies." Such a view attempts to provide for segre- 

 gation of characters without discussing vital units. 



Among the chemists is Dewitz (1902) who experimented with 

 fly larvae (Lucilia Caesar) and found that while there was no 

 tyrosinase in very young larvae (one or two days old) older 

 larvae contained a considerable quantity. When the pupae 

 formed they rapidly became pigmented, but this pigmentation 

 could not go on without oxygen. 



Phisalix (1905) working with cockroach larvae, found that 

 the larvae when hatched were colorless, but within three hours 

 changed through grey and brown to black. Phisalix concludes 

 that tyrosin and tyrosinase exist in the embryo long before the 

 color develops and that "it is probable that they coexist in the 

 egg or that they are deposited at the time of ovogenesis. ' ' 



Roques (1909) found that during the metamorphosis of 

 Limnophilus flavicornis Fabr., the amount of tyrosinase in the 

 body was greatest just before pigmentation began and as the 

 amount of pigment increased, the tyrosinase decreased, until it 

 was entirely absent when the beetle was fully pigmented. 



Riddle (1909, p. 329) reviews our knowledge of melanin 

 color formation and urges the futility of piling up factors one 

 for every color in the germ to explain the production of 

 melaninic color since without them " in an animal that pro- 

 duces melaninic color, there exists all the machinery necessary 

 to produce a series or scale of these colors." He continues (p. 

 336) : "This means that the animal that transmits the enzyme 



