146 THE FOUETH DAY. [CHAP. 



FIG. 48. 



A. HEAD OF AN EMBRYO CHTCK OF THE FOURTH DAT VIEWED FROM BELOW 



AS AN OPAQUE OBJECT. (Chromic acid preparation). 



CH. cerebral hemispheres. FB. vesicle of the third ventricle. Op. eyeball. 

 nf. naso-frontal process. M. cavity of mouth. S. M. superior maxillary 

 process of .P. i, the first visceral fold (inferior maxillary process). F. 2, F. 3. 

 second and third visceral folds. N. nasal pit. 



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

 third and fourth visceral iolds. In the section e thus made, are seen the 

 alimentary canal al with its collapsed walls, the neural canal n.c., the notochord 

 ch., the dorsal aorta AO., and the vertebral veins V. 



The incision has been carried just below the upper limit of the pleuroperi- 

 toneal cavity, consequently a portion of the somatopleure appears at the angle 

 between the two third visceral folds. Almost embraced by the piece of somato- 

 pleure is seen the end of the bulbus arteriosus Ao. 



In the drawing the nasal groove has been rather exaggerated in its upper 

 part. On the other hand the lower part of the groove where it runs between 

 the superior maxillary process S.M. and the broad naso-frontal process was, in 

 this particular embryo, extremely shallow and indeed hardly visible. 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. Letters as before. 



other, and helps to form the inner wall of each of them. 

 Abutting on the outer side of each groove and so helping to 

 form the outer wall of each, lie the ends of the superior 

 maxillary processes of the first visceral arch (Fig. 48 B, sm), 

 which like the fronto-nasal process are increasing in size. By 

 their continued growth, the groove is more and more deepened, 

 and leading as it does from the nasal pit to the cavity of the 

 mouth, may already be recognized as the rudiment of the 

 passage of the posterior nares. 



