I30 



BIOLOGICAL LECTURES. 



The embryo is viewed obliquely from the right side. The rudi- 

 ments of several organs — optic vesicles, branchial pouch, etc. 

 — have appeared upon the lateral walls of the head. Direct- 

 ing our attention to the margin of the nearest neural fold, we 

 note that it is clearly segmented throughout the head-region, 

 and backwards into the trunk to the point where, in the figure, 

 it disappears behind the yolk. The metameres extend, in 

 reality, to the posterior limit of the body. 



The next stage to be considered, Fig. 5, is immediately after 



^^ « /'-'' /-o ,p 



11/ < / 



i9 



Fig. 5. — Head of embryo of Acanthias after closure of the neural groove but before the formation 

 of the auditory vesicle. Reference marks as in the previous figure. 



the closure of the neural groove and before the auditory vesicle 

 has made its appearance. Certain anatomical landmarks of the 

 head-region have become established, and it is now possible to 

 determine the relation of the neural segments to other cephalic 

 structures. 



It is to be noted that the metameres of the fore- and mid- 

 brain are still visible from surface views, and almost immedi- 

 ately become indistinguishable in these regions, but continue 

 to be well defined in the hind-brain. There are five segments 

 well shown in the combined fore- and mid-brains. Three of 

 these probably belong to the fore-brain and two to the mid- 

 brain. All the embryos so far described exhibit the segmental 

 folds to the anterior end of the head, and a definite answer is 

 returned by these observations to the fundamental question 

 already proposed : How far forward does the segmentation 

 extend.? In view of the facts, we are justified in concluding 



