372 



HISTOLOGY 



h. 



mesodermal element of the skin below the epithelium which takes no 



part in its formation. This structure is the 

 cy ' * 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- 



G. 339- Parts of two longitudi- -v n ,. u- 1.1 



nal plates from a permanent SUC, Or CUtlS, changes from the flat, highly 

 feather somewhat more advanced specialized COnnectivC-tisSUC Cells to TOUnded 



mesodermal cells with oval nuclei whose 



in development than in the pre- 

 ceding figure. One of these 

 plates of cells will produce 



chromatin is arranged in a pattern that re- 



single barb with its double row 11 ,1 , ,v i 



of barbuies. cy.i., cylinder layer sembles the arrangement in the nuclei or 

 of ceils; b., cells which will form 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- 



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.) 



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 



