No. 3-] THE VERTEBRATE HEAD. 52 I 



produced faint surface indications in the median parts of the 

 medullary plate. 



Taking a considerable step forwards in the history of these 

 segments, we come to the condition represented in Fig. 31. 

 This figure shows a stage in which the medullary folds have 

 attained a nearly vertical position ; they are about to bend 

 towards each other and meet in the median plane, but, as yet, 

 they have not become approximated in any part of their course, 

 and, therefore, we have an open neural groove (see cut 9, p. 553) 

 extending the whole length of the embryo. In this figure the 

 embryo is viewed obliquely from the right side. The rudiments 

 of several organs have now appeared upon the head ; the most 

 anterior of these organs is the primary optic vesicle ; just back 

 of this, near the margin of the head-folds, is seen a similar 

 elevation that represents the combined vesicle of the mid-brain 

 and the first accessory optic vesicle (see p. 556). Still further 

 back in the same line, is another similar but smaller elevation, 

 which, I think, represents the second accessory optic vesicle. 

 Behind the latter structure the margin of the medullary fold is 

 bent abruptly downwards ; this is a normal condition in this 

 stage, and is of course found in the earlier stages. Back of the 

 primary optic vesicle and somewhat between it and the mid- 

 brain vesicle, is a rounded eminence which is the external indi- 

 cation of the mandibular cavity, and behind this is the 

 branchial pouch from which the branchiae are subsequently 

 formed. The front end of the gastrular cavity is being cut off 

 along with the increase in the head flexure. 



Directing our attention to the margin of the right medullary 

 fold, we note that it is clearly segmented through the head re- 

 gion, and backwards into the trunk region, where, in the figure, 

 it disappears behind the yolk. The metameres extend in reality 

 to the posterior part of the embryo. There has been a slight 

 change in position of the foremost segments with reference to 

 the rest of the head-plate. The three anterior ones are no 

 longer, as in Fig. 29, on a part of the margin that looks for- 

 wards, but they have been shifted backwards, and that part of 

 the margin that was anterior, now constitutes a part of the 

 lateral border. Of course this shifting of position is brought 



