STRUCTURE AND DEVELOPMENT OE LEPIDOSTEUS. 
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of the fore-brain. Their outer ends are dilated, and are in contact with the external 
skin. The formation of the optic cup has not, however, commenced. The nervous 
layer of the skin adjoining the outer wall of the optic cup is very slightly thickened, 
constituting the earliest rudiment of the lens. 
In one of our embryos of this day the developing auditory vesicle still has the form 
of a pit, but in the other it is a closed vesicle, already constricted off from the nervous 
layer of the epidermis. 
With reference to the development of the excretory duct we cannot add much to 
what we have already stated in describing the last stage. 
The duct is considerably dilated anteriorly (Plate 23, fig. 31, sg.); but our sections 
throw no light on the nature of the abdominal pore. The posterior part of the duct 
has still the form of a hollow ridge united with somatic mesoblast (Plate 23, fig. 32, sg.). 
During this stage, the embryo becomes to a small extent folded off from the yolk- 
sac both in front and behind, and in the course of this process the anterior and. 
posterior extremities of the alimentary tract become definitely established. 
We have not got as clear a view of the process of formation of these two sections of 
the alimentary tract as we could desire, but our observations appear to show that the 
process is in many respects similar to that which takes place in the formation of the 
anterior part of the alimentary tract in Elasmobranchii.One of us has shown that 
in Elasmobranchs the ventral wall of the throat is formed not by a process of folding 
in of the hypoblastic sheet as in Birds, but by a growth of the ventral face of the 
hypoblastic sheet on each side of and at some little distance from the middle line. 
Each growth is directed inwards, and the two eventually meet and unite, thus 
forming a complete ventral wall for the gut. Exactly the same process would seem 
to take place in Lepidosteus, and after the lumen of the gut is in this way established, 
a process of mesoblast on each side also makes its appearance, forming a mesoblastic 
investment on the ventral side of the alimentary tract. Some time after the ali¬ 
mentary tract has been thus formed, the epiblast becomes folded in, in exactly the same 
manner as in the Chick, the embryo becoming thereby partially constricted off from 
the yolk (Plate 23, figs. 33, 34). 
The form of the lumen of the alimentary tract differs somewhat in front and behind. 
In front, the hypoblastic sheet remains perfectly flat during the formation of the 
throat, and thus the lumen of the latter has merely the form of a slit. The lumen of 
the posterior end of the alimentary tract is, however, narrower and deeper (Plate 23, 
figs. 33, 34, al.). Both in front and behind, the lateral parts of the hypoblastic sheet 
become separated from the true alimentary tract as soon as the lumen of the latter is 
established. 
It is quite possible that at the extreme posterior end of the embryo a modification 
of the above process may take place, for in this region the hypoblast appears to us to 
have the form of a solid cord. 
* F. M. Balfour, ‘ Monagrapli on the Development of Elasmohranch Fishes,’ p. 87, plate ix., fig. 2. 
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