DEVELOPMENT OF THE CHICK 3TIKST PERIOD. 



303 



volucra of the nervous centres ; the brain, and medulla ob- 

 longata, up to this 

 time, are, therefore, 

 in fact, shut vesicles, 

 which, on account of 

 their transparency 

 only, appear as open 

 spaces lying between 

 the sinuous cristse of 

 the dorsal laminae. 

 Outwardly, from the 

 cristae of the dorsal 

 laminae, and the four- 

 cornered laminae of 

 the vertebral arches, 

 proceeds the serous 

 lamina of the germi- 

 nal membrane, thick- 

 ening as it grows, 

 and bending from 

 both sides at the 

 same time slightly 

 inwards ; in this part 

 a number of small 

 dark leaflets make 

 their appearance si- 

 multaneously, which 

 become particularly 

 plain in the trans- 

 verse section (fig. 

 335, A, and especi- 

 ally fig. 338, A, b 2 )- 

 these are the rudi- 

 ments of the trans- 

 verse processes of 

 the vertebrae, and, 

 farther out, of the 



Fig. 334. Magnified view of the area pellucida of the vitellus, fig. 329 

 b, b, crests of the dorsal laminae, receding from each other anteriorly 

 to form the cerebral cells ; d l , cell of the eyes and thalami ; d-, cell of the 

 corpora quadrigemina ; d 3 , cell of the medulla oblongata ; c, c, c, c, laminre 

 dorsales, of which ten are present on either side ; e, anterior fold of the 



