344 H. B. POLLARD. 



the posterior thickened ingrowth, which is also continuous 

 with the ectoderm of the lens^ gives rise to the portion of the 

 mouth lying in the region of the articulation of the jaws. 



Considering now the endoderm at this stage, it is found 

 that the alimentary canal is only tubular in the auditory 

 region of the head. The lumen extends anteriorly to a short 

 distance behind the termination of the chorda dorsalis, then 

 passes outwards to open at the only gill- slit formed as yet. 

 This gill-slit is the hyoid, or the aperture behind the 

 subsequently formed operculum. By German authors it is 

 termed the " Kiemendeckelspalte." In front of the chorda 

 the outlines of the endoderm are almost impossible to 

 discover, but it appears to extend forward as a solid layer, 

 becoming in the median line very thin and unilaminar. 

 Laterally, however, in front a more distinct prolongation 

 meets the posterior ingrowth of ectoderm, and fuses with it, 

 as shown already in figs. 9 and 10. Curiously enough, when 

 this prolongation meets the ectoderm it also proceeds upward, 

 and fuses with the mesoderm surrounding the eye. 



In fig. 2 these conditions are represented diagrammatically. 

 On the right of the figure only the brain and eyes and a 

 portion of the premandibular mesoderm are drawn. On the 

 left, ectoderm and endoderm are intact. The embryo is 

 supposed to be viewed from the ventral side, that is through 

 the yolk. The edge of the ectodern, where it passes over the 

 yolk, is cut in section. The blue tint shows its extent. 

 Towards the centre of the embryo, where the ectodermic layer 

 is one cell thick, it passes without demarcation into the 

 corresponding endodermal layer. The dotted blue line indi- 

 cates the extent of the above-described thickened ingrowth of 

 the ectoderm. This figure also shows how these ingrowths are 

 moulded to the shape of the brain and eyes. Consequently 

 the doubleness of origin must probably be regarded merely as 

 an embryonic feature without any phylogenetic significance. 



The endoderm is tinted yellow. Posteriorly beneath the 

 chorda the lumen is indicated^ and the dotted line proceeding 

 to the hyoid cleft {Hy.) shows the communication of this 



