STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 753 



in this region the hypoblast appears to us to have the form of 

 a solid cord. 



We could detect no true neurenteric canal, although a more 

 or less complete fusion of the germinal layers at the tail end of 

 the embryo may still be traced. 



During this stage the protoplasm of the notochordal cells, 

 which in the last stage formed a kind of axial rod in the centre 

 of the notochord, begins to spread outwards toward the sheath 

 of the notochord. 



Eighth day after impregnation. The external form of the em- 

 bryo (Plate 34, fig. 9) shews a great advance upon the stage last 

 figured. Both head and body are much more compressed later- 

 ally and raised from the yolk, and the head end is folded off for 

 some distance. The optic vesicles are much less prominent 

 externally. A commencing opercular fold is distinctly seen. 

 Our figure of this stage is not, however, so satisfactory as we 

 could wish. 



A thickening of the nervous layer of the external epiblast 

 which will form the lens (Plate 36, fig. 35, /.) is more marked 

 than in the last stage, and presses against the slightly concave 

 exterior wall of the optic vesicle (op.). The latter has now 

 a large cavity, and its stalk is considerably narrowed. 



The auditory vesicles (Plate 36, fig. 36, au.} are closed, ap- 

 pearing as hollow sacks one on each side of the brain, and are no 

 longer attached to the epiblast. 



The anterior opening of the segmental duct can be plainly 

 seen close behind the head. The lumen of the duct is consider- 

 ably larger. 



The two vertebral portions of the mesoblast are now sepa- 

 rated by a considerable space from the epiblast on one side and 

 from the notochord on the other, and the cells composing them 

 have become considerably elongated from side to side (Plate 36, 

 fig. 37, ins.). 



In some sections the aorta can be seen (Plate 36, fig. 37, ao.) 

 lying close under the sub- notochordal rod, between it and the 

 hypoblast, and on either side of it a slightly larger cardinal vein 

 (cd. v.). 



The protoplasm of the notochord has now again retreated 

 towards the centre, shewing a clear space all round. This is 



