DEVELOPMENT OF THE CHICK SECOND PERIOD. 309 



history of the development, as, from the general vigour of the 

 formative processes, all the organs now begin to be evolved, 



Fig. 339. View of an embryo, four lines long, magnified about eight 

 diameters. The embryo is seen from the abdominal surface ; the time is 

 the middle of the third day. a, Area pellucida ; b, anterior cerebral cell 

 (the hemispheres) ; c, cell of the thalami and crura cerebri ; d, corpora 

 quadrigemina ; e, cerebellum and medulla oblongata ; /, the eye, a wide 

 cleft inferiorly ; g, the auditory vesicle lying in front of the medulla 

 oblongata ; h, A, h, vertebral lamina ; i, ventricle of the heart ; k, atrium 

 cordis ; Ar 1 , superior, and 2 , inferior vein of the blastoderma ; I, bulb of 

 the aorta, giving off the four branchial arteries, over which lie three 

 branchial arches, 1, 2, 3 ; m, m, arteries of the blastoderma proceeding 

 from the divided trunk of the aorta ; inwards from either aorta the bodies 

 of the vertebral laminae are united by suture ; n, the allantois just 

 budding forth ; o, o, o, o, margins of the abdominal cavity, reflected su 

 periorly into the involucrum capitis, p ; inferiorly into the involucrum 

 caudae, g, q. The mesentery, Wolffian bodies, &c., which have by this time 

 began to appear, are left out. The actual length of the embryo is indicated 

 by the line with the asterisk. 



