744 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



After the removal of the egg-membranes described above 

 we find that there remains a delicate membrane closely attached 

 to the epiblast. This membrane can be isolated in distinct 

 portions, and appears to be too definite to be regarded as an 

 artificial product. 



We have been able to prepare several more or less complete 

 series of sections of embryos of this stage (Plate 35, figs. 18 22\ 

 These sections present as a whole a most striking resemblance 

 to those of Teleostean embryos at a corresponding stage of 

 development. 



Three germinal layers are already fully established. The 

 epiblast (ep.} is formed of the same parts as in Teleostei, viz. : 

 of an outer epidermic and an inner nervous or mucous stratum. 

 In the parietal region of the embryo these strata are each 

 formed of a single row of cells only. The cells of both strata 

 are somewhat flattened, but those of the epidermic stratum are 

 decidedly the more flattened of the two. 



Along the axial line there is placed, as we have stated 

 above, the medullary plate. The epidermic stratum passes over 

 this plate without undergoing any change of character, and 

 the plate is entirely constituted of the nervous stratum of the 

 epidermis. 



The medullary plate has, roughly speaking, the form of a 

 solid keel, projecting inwards towards the yolk. There is no 

 trace, at this stage at any rate, of a medullary groove ; and as 

 we shall afterwards shew, the central canal of the cerebro-spinal 

 cord is formed in the middle of the solid keel. The shape of 

 this keel varies according to the region of the body. In the 

 head (Plate 35, fig. 18, m.c.}, it is very prominent, and forming^ 

 as it does, the major part of the axial tissue of the body, impresses 

 its own shape on the other parts of the head and gives rise to 

 a marked ridge on the surface of the head directed towards the 

 yolk. In the trunk (Plate 35, fig's. 19, 20) the keel is much less 

 prominent, but still projects sufficiently to give a convex form 

 to the surface of the body turned towards the yolk. 



In the head, and also near the hind end of the trunk, the 

 nervous layer of the epiblast continuous with the keel on each 

 side is considerably thicker than the lateral parts of the layer. 

 The thickening of the nervous layer in the head gives rise to 



