General Development of the Trunk from Stage H to the Close of Embryonic Life.
External Epiblast.
The change already alluded to in the previous chapter (p. [317]) 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. 13, fig. 8), unquestionably pertaining to the epiblast. This layer is especially thickened in the terminal parts of the paired fins (Pl. 13, 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 (Pl. 13, 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 variations 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 those of Dr O. Hertwig[228], that it is not necessary to record them. The so-called enamel layer is a simple product of the thickening and calcification of the basement membrane, and since this membrane is derived from the mucous layer of the epidermis, the enamel is clearly to be viewed as an epidermic product. There is no indication of a gradual conversion of the bases of the columnar cells forming the mucous layer of the epidermis into enamel prisms, as is frequently stated to occur in the formation of the enamel of the teeth in higher Vertebrates.
Lateral line.
The lateral line and the nervous structures appended to it have been recently studied from an embryological point of view by Götte[229] in Amphibians and by Semper[230] in Elasmobranchii.