118 THE THIRD DAY. [CHAP. 



37, N). Like the lens and the labyrinth of the ear, they are 



HEAD OF AN EMBRYO CHICK OF THE THIRD DAY VIEWED SIDEWAYS AS AN 

 OPAQUE OBJECT. (Chromic acid preparation.) 



C.H. Cerebral hemispheres. F. B. Vesicle of third ventricle. M.B. Mid-brain. 

 Cb. Cerebellum. H.B. Medulla oblongata. 



N. Nasal pit. ot. otic vesicle in the stage of a pit with the opening not yet 

 closed up. op. Optic vesicle, with I. lens and ch.f. choroidal fissure. The 

 small dot in the centre of the lens indicates the remnant of its external 

 opening. The superficial epiblast moulds itself to the form of the optic 

 vesicle and the lens ; hence the choroidal fissure, though formed entirely 

 underneath the superficial epiblast, is distinctly visible from the outside. 



T F. The first visceral fold ; above it is seen a slight indication of the superior 

 maxillary process. 



2, 3, 4 F. Second, third and fourth visceral folds, with the visceral clefts 

 between them. 



SO. Portion of the somatopleure springing from between the ends of the 

 visceral folds. 



formed from the external epiblast; unlike them they are 

 never closed up. At first they have no distinct connection 

 with their respective olfactory vesicles, and their openings 

 are independent and separate, there being as yet no actual 

 mouth to connect them with each other. 



14. It must be borne in mind that especially in the 

 early stages of development, owing, to the very unequal 

 growth of different parts, the relative position of the various 

 structures is continually shifting. This is very well seen in 

 the instance of the heart. At its first appearance, the heart 

 is lodged immediately beneath the extreme front of the ali- 

 mentary canal so far forwards as to underlie that portion of 

 the medullary canal which will form the brain. It is, in 

 fact, at that epoch a part of the head. From that early 

 position it gradually recedes farther and farther backward, 

 until, at the end of the third day, a considerable interval is 



