SECRETORY PHENOMENA IN SILK GLAND. 42! 



region. To strengthen the cell against the resulting pressure 

 and to prevent it from distortion, a thin layer of hyalin material 

 might be conceived to serve as a support. At the point in the 

 tube where it begins to narrow anteriorly, the pressure becomes 

 greater and more support is necessary. That fibrillae should 

 appear in the transition region between the middle and anterior 

 regions, is, therefore, to be expected. It is quite possible, 

 moreover, that these fibrillae may be more than merely supporting 

 structures. Their shape and position around the lumen seems 

 to indicate that they may have a contractile function, serving, 

 perhaps, as a simple musculature in pushing the products of 

 secretion into the long conducting tube. It is evident that some 

 regulatory mechanism is necessary to prevent the small tube 

 from becoming clogged by the ever accumulating secretion, and 

 likewise, to regulate the amount of material passing through in 

 order to produce a silk thread of uniform diameter. 



4. The Secretion. 



(a) Historical. The morphological appearance of the secreted 

 material has received considerably more attention throughout 

 the literature than the mechanism of the secretion process, and 

 yet little is definitely known about the real nature of the secreted 

 substance. Staining reactions have been used almost exclusively 

 as criteria upon which to base conclusions. Little detailed work 

 has been done upon the actual chemical composition of the silk 

 fiber further than a general analysis of the chemical components, 

 the result of which has been to show that the fiber is made up of 

 two slightly different substances; an outer, acidophilic layer of 

 sericin, and a central core of basophilic fibroin. The former is 

 quite similar in composition to the latter except that, according 

 to Blanc ('89), it contains more oxygen, dissolves more readily in 

 alkaline solutions and is more granular in appearance. 



Tanaka ('n, p. 18-20) has reviewed very completely the 

 results of previous workers and has arranged their views con- 

 cerning the origin of fibroin and sericin into four principal 

 groups. He then attempts to show that these theories have 

 failed to satisfy all the conditions and proceeds to modify and 

 combine parts of each theory into one more in accord with 



