752 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



of a hollow ridge united with somatic mesoblast (Plate 36, fig. 

 32, sg.). 



During this stage, the embryo becomes to a small extent 

 folded off from the yolk-sack 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 shew 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 1 . 

 One of us has shewn that in Elasmobranchs the ventral wall of 

 the throat is formed not by a process of folding in of the hypo- 

 blastic 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 

 36, figs. 33, 34). 



The form of the lumen of the alimentary tract differs some- 

 what 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 36, figs. 33, 34, a/.). 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 



1 F. M. Balfour, "Monograph on the Development of Elasmobranch Fishes," 

 p. 87, plate 9, fig. 2. [This edition, p. 303, pi. 10.] 



