180 



THE FIFTH DAY. 



[CHAP. 



Fig. 56. 



F.B 



mx: 



A. Head of an Embkto Chick of the Foueth Day viewed fkom below 

 AS AN opaque object. (Chromic acid preparation.) 



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

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

 process of F. 1, 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 folds. In the section e thus made, are seen the 

 alimentary canal al with its collapsed walls, the neural canal m.c, the noto- 

 chord ch. , the dorsal aorta A 0., 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. 



process a rather bilobed appearance. These projecting portions 

 of the fronto-nasal process form on each side the inner 

 margins of the rapidly deepening nasal grooves, and are 

 sometimes spoken of as the inner nasal processes. The outer 

 margin of each nasal groove is raised up into a projection 

 frequently spoken of as the outer nasal jwocess which runs 

 downwards to join the superior maxillary process, from which, 



