126 



THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



the former series will be found very instructive in supplementing the following 

 descriptions. 



Transverse Section through the Eyes and Otocysts (Fig. 129). — The brain is 

 sectioned twice, lengthwise through the myelencephalon, transversely through the fore-brain. 

 The brain wall shows difierentiation into three layers: (1) an inner ependymal layer densely 

 cellular; (2) a middle mantle layer of nerve cells and fibers; (3) an outer marginal layer chiefly 

 fibrous. These same three layers are developed in the spinal cord. A thin vascular layer 

 differentiated from the mesenchyma surrounds the brain wall and is the anlage of the pia 

 mater. The myelencephalon shows three neuromeres in this section. The telencephalon is 



Foiirlh renlriih 



Gang, jugular e n. m 



Gang, aciisl. n. S 



Mandibular ramus ». i 

 M axillary ramus n. i 



Ant. lobe hypophysis 

 Lens vcsiiic 

 Third ventricle of dicnccphal ii 



Lai. ventricle of Icknccphalon 



n all of myelencephalon 

 \ accessorius 



X . glossopharyngeus 



Olocysl 



Cinii;. geniculat. n. 7 

 I 4— -V. obdurens 

 Bdsihir artery 

 Sinus idwrnosus 

 I III. caiotid artery 



Opiic vesicle 



Foramen interventricutare 



Fig. 129. — Transverse section passing through the eyes and otocysts of a 10 mm. pig embryo. X 22.5. 



represented by the paired cerebral hemispheres, their cavities, the lateral ventricles, connecting 

 through the interventricular foramina with the third ventricle of the diencephalon. Close to the 

 ventral wall of the diencephalon is a section of the anterior lobe of the hypophysis (Rathke's 

 pocket) near which are the internal carotid and basilar arteries. Lateral to the diencephalon. 

 is the optic cup and lens vesicle of the eye, which are sectioned caudal to the optic stalk. The 

 outer layer of the optic cup forms the thin pigment layer; the inner thicker layer, is the nervous 

 layer of the retina. The lens is now a closed vesicle distinct from the overlying corneal 

 ectoderm. 



The large vascular spaces are the cavernous sinuses, which drain by way of the vv. capitis 

 laterales into the internal jugular veins. Transverse sections may be seen of the mMcillary 



