302 



EMBRYOLOGY 



3 



to twelve "pairs may be reckoned (fig. 334, c, c, c). At 

 this time the dorsal laminae separate still more from one 

 another in front, so that many spaces or cells become dis- 

 tinctly visible be- 

 tween them ; the 

 largest or most 

 anterior of these 

 cells (fig. 334, d) 

 has become some- 

 what pointed for- 

 wards, and curved 

 underneath ; late- 

 rally it presents 

 wide bending in- 

 lets, which indi- 

 cate the first for- 

 mation of the 

 eyes ; it is the 

 cell of the thala- 

 mi and crura of 

 the cerebrum ; 

 the second small- 

 er cell (d 2 ) is the 

 cell of the cor- 

 pora quadrigemi- 

 na ; the third, an 

 elongated cell 

 (</ 3 ), belongs to 

 the medulla ob- 

 longata. The 

 transparent mass 

 of the brain and 

 spinal cord ac- 

 quires greater 

 consistency, and 



is covered with a 

 f ijr. 333. Yolk of the natural size after thirty- fi , , . , , 



six hours of incubation ; a, yolk ; b, fiddle-shaped 



I ellucid area, in the middle of which the embryo transparent lay- 

 is seen. In the vascular area, c, c, the insulae er, the future 

 sanguinis, or blood islets, begin to appear. membranous ill- 



Fig. 332. Ideal sections of fig. 331. A, 

 transverse section ; B, longitudinal section. In 

 A,/, section of the vertebral laminae. In B, 

 formation of the head by the reflection of the 

 blastoderma ; e, margin of the involucrum capitis, 

 and entrance into the,future intestinal canal (fovea 

 cardiaca of Wolff). The other references are the 

 same as in fig. 329. 



