372 



HISTOLOGY 



cy.l. 



h. 



FIG. 339. Parts of two longitudi- 

 nal plates from a permanent 

 feather somewhat more advanced 

 in development than in the pre- 

 ceding figure. One of these 

 plates of cells will produce a 

 single barb with its double row 

 of barbules. cy.l., cylinder layer 

 of cells; b., cells which will form 

 the barbules; h., horny covering. 

 (After DAVIES.) 



mesodermal element of the skin below the epithelium which takes no 

 part in its formation. This structure is the 

 scale found on the body surface of the tele- 

 ost fish. 



The skin of a fish consists of a thick 

 layer of connective tissue laid down in thin 

 horizontal layers and forming the base on 

 which rests a heavy, stratified epithelium. 

 In the embryo these layers are distinctly 

 separated from one another by a clear, 

 sharp-cut line, the basement membrane. 

 The outermost layer of the connective tis- 

 sue, or cutis, changes from the flat, highly 

 specialized connective-tissue cells to rounded 

 mesodermal cells with oval nuclei whose 

 chromatin is arranged in a pattern that re- 

 sembles the arrangement in the nuclei or 

 epithelial cells. This layer slowly increases 

 by mitotic divisions. Later in the develop- 

 ment (trout of forty-five millimeters), it can 

 be seen that this layer has become divided into two layers by a deposit 

 which the cells are 

 laying down be- 

 tween them (Fig. 

 340). This layer of 

 homogeneous and 

 dense material rep- 

 resents a longitudi- 

 nal section of a thin 

 oval plate called the 

 scale. It increases 

 in size (which is 

 represented by 

 length in our draw- 

 ing) and also in 

 thickness. 



These scales oc- 

 cur, placed in a reg- 

 ular pattern, over 

 most of the fish's in- 

 tegument. At first 

 they do not interfere with one another, but later they increase so in 

 size, especially on the posterior edge, that they overlap, and come to lie 



conn. t. 



FIG. 340. Several basal epithelial cells (b.c.) resting on the rudi- 

 ment of a young scale of an embryo trout, conn.t., unmodified 

 connective tissue of the corium; /., the two distal layers of con- 

 nective tissue constituting the scale follicle. The young scale, 

 in section, is indicated by a dotted line. (After NUSBAUM.) 



