No. 3-] THE VERTEBRATE HEAD. 55 I 



Fig. 5 is engraved from a photograph in which the optic vesicles 

 show very well. The specimen from which the photograph was 

 made showed about six mesoblastic somites, and was 2^^ mm. 

 in length. The circular areas may be made out satisfactorily 

 in slightly earlier stages, in which only three mesoblastic somites 

 are differentiated, and in which the neural folds of both head 

 and trunk are not only broadly expanded, but are even ventrally 

 curved. The circular depressions are at first separated from 

 one another by a raised welt. This has already been spoken 

 of in Part I as a tongue-like process extending from the median- 

 anterior tip backwards to two-thirds the length of the cephalic 

 plate. It continues to be a prominent feature of the cephalic 

 plate for some time. I can offer no suggestion as to its signifi- 

 cance, outside of the obvious suspicion that it may represent a 

 proboscis of some kind, or that it may be related to the large 

 notochord of this region. On this point I have no conjectures to 

 make. The depression to form the infundibulum starts a little 

 after the first appearance of the optic vesicles, and cuts the 

 median process in two, so that the anterior tip is separated from 

 the rest, and the depression for optic vesicles and infundibulum 

 combined reaches across the median plane of the cephalic 

 plate (Figs. 6, 7, 8, 9, etc). The optic vesicles, however, do 

 not reach to the lateral margins of the neural folds ; the latter 

 are much expanded beyond them. 



It is evident that we cannot, in a strict sense, speak of 

 such vesicles as "diverticula of the fore-brain." The "diver- 

 ticula" are found before the fore-brain. The depressions once 

 started grow deeper and press outward laterally ; when fully 

 formed they are cupped, almost rounded in outline, concave 

 from within, and they form rounded elevations where they come 

 in contact with the outer layer of epiblast. Figs. 6, 7, 8, 9, 10, 

 show very well the appearance presented by these primary 

 optic vesicles when viewed from above. Fig. 7 is magnified 

 higher than the others, and is, therefore, larger, but the others 

 are all uniformly enlarged. These figures all show the interior 

 of the optic pits. They represent a range of stages only slightly 

 older than that shown in Fig. 5. In all of them the depression 

 in which the optic vesicle lies extended across the median 



