CHAPTER VII. 



Genekal Development of the Trunk from Stage H 

 TO THE Close of Embryonic Life. 



External Epihlast. 



The change already alluded to in the previous chapter (p. 9D) 

 by which the external epiblast or epidermis becomes divided 

 into two layers, is completed before the close of stage L. 



In the tail region at this stage three distinct strata may be 

 recognized in the epidermis. (1) An outer stratum of flattened 

 horny cells, which fuse together to form an almost continuous 

 membrane. (2) A middle stratum of irregular partly rounded 

 and partly flattened cells. (3) An internal stratum of columnar 

 cells, bounded towards the mesoblast by a distinct basement 

 membrane (PL XIL fig. 8), unquestionably pertaining to the 

 epiblast. This layer is especially thickened in the terminal parts 

 of the paired fins (PI. xii. fig. 1). The two former of these 

 strata together constitute the epidermic layer of the skin, and 

 the latter the mucous layer. 



In the anterior parts of the body during stage L the skin 

 only presents two distinct strata, viz. an inner somewhat 

 irregular layer of rounded cells, the mucous layer, and an outer 

 layer of flattened cells (PI. XIL fig. 8). 



The remaining history of the external epiblast, consisting as 

 it does of a record of the gradual increase in thickness of the 

 epidermic strata, and a topographical description of its varia- 

 tions in structure and thickness in different parts, is of no 

 special interest and need not detain us here. 



In the late embryonic periods subsequent to stage Q the 

 layers of the skin cease to be so distinct as at an earlier period, 

 partly owing to the innermost layer becoming less columnar, 

 and partly to the presence of a large number of mucous cells, 

 which have by that stage made their appearance. 



I have followed with some care the development of the 

 placoid scales, but my observations so completely accord with 



