SAGITTAL SECTIONS OF EMBRYO OF 12 MM. 



209 



where the epithelium lining the sinus becomes continuous with the epidermis. 

 Cephalad from the sinus, but close to it, lies a small dark rounded mass, the 

 anlage of the thymus gland (compare Fig. 118, Thm). The thymus anlage 



o/. 



G.petr. 



G.nod. 



Cen<. S. 



N.12. 



j»g-"" 



Fig. 124. — Pig, 12.0 mm. No. 7. Sagittal Section 25. 

 Ac.F, Acustico-facial ganglion complex. Aur, Auricle of the heart. Cerv.S, Diverticulum of the cervical 

 sinus, just in front of which shows the anlage of the thymus, which is deeply stained. Cerv.6, Sixth cervi- 

 cal nerve. C(s, Coelom around the heart or pericardial cavity. G.j'ug, Ganglion jugulare of the vagus 

 nerve. G.nod, Ganglion nodosum of the vagus nerve. G.petr, Ganglion petrosum of the glossopharyngeal 

 nerve. G.iri, Ganglion of the trigeminus nerve. H, Lateral wall of the cerebral hemisphere. Jug, 

 Jugular vein. {Jug^, Behind the trigeminus. Jug'' , Branch in front of the trigeminus. Jug'" , Main 

 stem behind the vagus. Jug"" , Main stem descending to join the duct of Cuvier. ) m. An undetermined 

 structure, probably the anlage of a lingual muscle. Md, Mandible. N.^, Root of the fifth or trigeminal 

 nerve. N.op, Optic nerve. N.12, Twelfth or hypoglossal nerve. Ot, Otocyst. Ph, Pharynx. R.L,. 

 Recessus lateralis of the fourth ventricle. Ve, Small branch of the jugular vein. Vent, Ventricle of the 

 heart. X 22 diams. 



is produced by proliferation of the entodermal cells on the anterior side of 

 the third cleft, and is penetrated by blood-vessels which seem to be sinusoids, 

 although their history has not been worked out. The great vein of the head^ 



14 



