276 



STUDY OF PIG EMBRYOS. 



the side of the cleft toward the head. This structure is the anlage of the 

 nodulus thymicus and is already penetrated by small blood-vessels which are 

 perhaps not capillaries, but sinusoids. The fate of nodulus is uncertain; it 

 probably forms the head of the thymus, and not the carotid gland as some 



Fourth aorti 



FIG. 194. PIG, 12.0 MM. TRANSVERSE SERIES 5, SECTION 321. 



Fourth aortic arch. Art.v, Vertebral artery. AH, External auditory cleft. Card, Anterior cardinal vein. 

 cl.iii, Third internal gill-cleft. G.nod, Ganglion nodosum. Hy, Hyoid arch. L.V, Lateral ventricle. 

 .\'<i. Nasal pit. N.cerv.i, First cervical nerve. N.ceru.2, Second cervical nerve. N.II, Spinal accessory 

 nerve. IV. 12, Hypoglossal nerve. N.od, Nodulus thymicus. Nv,2, Main trunk of second cervical nerve. 

 Ol.pl, Olfactory plate. R.ex.n, Ramus exter,nus of the spinal accessory. Sp.c, Spinal cord. Th\r, 

 Thyroid. Tr, Trachea. X 22 diams. 



have suggested. Thfc student should clearly understand that the median region 



of the third gill-cleft is really the pharynx proper. From its median 



ventral line ' arises the beginning of the trachea, Tr, which shouU, perhaps, 



designated as the anlage of the larynx. The entoderm yxtends down 



! 



