128 



THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



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

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

 The brain wall shows differentiation into three layers: (i) 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 represented by the paired cerebral hemispheres, their cavities, the 



Fourth ventricle 



Gang, jugulare n. 10 



Gang, acusticum n. 8 



Mandibular ramus n. 5 

 Maxillary ramus n. 



Ant. lobe of hypophysis 

 Lens vesicle 



Wall of myelencephalon 



N. accessorius 



N. glossopharyngeus 



Olocysi 



Gang, geniculi. n. 7 

 N. abducens 

 Basilar artery 

 Sinus cavernosits 

 Inf. carotid artery 



'ptic vesicle 

 Foramen intervenlriculare 



Third ventricle of diencephalon' 



Lot. ventricle of telencephalon' 

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



lateral ventricles, connecting through the intervenlricular foramina with the third ventricle 

 of the diencephalon. Close to the ventral wall of the diencephalon is a section of the ante- 

 rior lobe of the hypophysis (Rathke's pouch) 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 pig- 

 ment 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 w. 

 capit : s laterales into thp internal jugular veins. Transverse sections may be seen of the 

 maxi n ary and mandibular branches of the n. trigeminus; the n. abducens is sectioned longi- 

 tudinally. The small nn. oculomotorius and trochlearis should be identified in sections 



