VII.] THE HEAD. 201 



FIG. 69. 



A. HEAD OF AN EMBRYO CHICK OF THE FOURTH DAY 



VIEWED FROM BELOW AS AN OPAQUE OBJECT. (Chromic 



acid preparation.) 



C.H. cerebral hemispheres. F.B. vesicle of the third ventricle 

 or thalamencephalon. Op. eyeball, nf. naso-frontal process. 

 M. cavity of mouth. S.M. superior maxillary process of F. 1, 

 the first visceral fold (mandibular arch). F. 2, F. 3 second 

 and third visceral arches. N. nasal pit. 



In order to gain the view here given the neck was cut across 

 between the third and fourth visceral folds. In the section e 

 thus made are seen the alimentary canal al, the neural canal n.c., 

 the notochord ch., the dorsal aorta AO., and the jugular veins V. 

 Ao. bulbus arteriosus. 



In the drawing the nasal groove has been rather exaggerated 

 in its upper part. On the other hand the lower and shallower 

 part of the groove where it runs between the superior maxillary 

 process S.M. and the broad naso-frontal process has not been 

 satisfactorily rendered. Hence the end of the superior maxillary 

 process seems to join the inner and not, as described in the text, 

 the outer margin of the nasal groove. A few hours later the 

 separation of the two would have been very visible. 



B. The same seen sideways, to shew the visceral folds, ot. otic 

 vesicle. Remaining letters as before. 



