192 



STUDY OF PIG EMBRYOS. 



original connection with the pharynx has been lost. The anlage is now isolated 

 from its parent germ-layer and is imbedded in mesenchyma. It is solid, for 



Fig. ii8. — Pig, 12.0 mm. Transverse Series 5, Section 321. 

 A0.4, Fourth aortic arch. Art.v, Vertebral artery. Au, External auditory cleft, cl.iii. Third internal gill 

 cleft. G nod, Ganglion nodosum. By, Hyoid arch. Jug, Jugular vein. L. V, Lateral ventricle. A^a, 

 Nasal pit. N.cerv.i, First cervical nerve. N.cerv.2, Second cervical nerve. N.ii, Spinal accessory 

 nerve. N.12, Hypoglossal nerve. Nv.2, Main trunk of second cervical nerve. Ol.pl, Olfactory plate. 

 R.ex.g, Ramus externus of the glossopharyngeus. Sp.c, Spinal cord. Thm, Thymus. Tkyr, Thyroid. 

 TV, Trachea. X 22 diams. 



the cavities of the thyroid follicles are not developed until considerably later. 

 Just above the third gill cleft may be seen a large, darkly stained mass, 



