336 STUDY OF PIG EMBRYOS. 



epithelium (Fig. 201), is now entirely separated from the mouth, and is an epithe- 

 lial vesicle with an irregular cavity. The epithelium has sent out, especially on 

 its anterior side, a number of solid outgrowths. The infundibular gland and 

 hypophysis constitute the pituitary body of the adult. They are surrounded by 

 loose mesenchymal tissue. The sella turcica, in which the pituitary body of the 

 adult is lodged, is marked out, because the chondrification, which is to form the 

 sphenoidal cartilages, has already begun about these structures. The sphenoidal 

 cartilage is continuous, on the one hand, with that of the nasal septum, Sept, and, 

 on the other, with that of the vertebral column, Vert. From the opening of the 

 infundibular gland the brain-wall ascends and joins the habenular arch, where it 

 suddenly thickens. The arch forms the floor of the mid-brain. The" roof of the 

 mid-brain, M.b, is quite thin, and forms the large arch in which the differentia- 

 tion of the anterior and posterior corpora quadrigemina is not yet shown. At its 

 posterior boundary the wall of the roof of the mid-brain bends inward, marking 

 the constriction of the so-called isthmus. We now reach the cavity, Ven.IV, or 

 fourth ventricle, of the hind-brain. This cavity is subdivided into an anterior and 

 a posterior portion. The boundary is marked on the dorsal side by the inward 

 projection of the ependymal roof of the ventricle to form the choroid plexus, Plx. 

 IV, and on the ventral side by the angle formed by the union of the medulla 

 oblongata, Md.ob, with the vertical peduncles of the brain. The peduncles, con- 

 tinuing upward, join the habenular arch. In front of the choroid plexus the arch- 

 ing brain-wall, Cbl, represents the median anlage of the cerebellum. The lateral 

 portions of the cerebellum are much thicker. Behind the choroid plexus the roof, 

 Epen, of the fourth ventricle is very thin. The medulla oblongata, Md.ob, is a 

 thick mass of tissue which passes over abruptly into the spinal cord. The spinal 

 cord is cut, as a whole, somewhat obliquely. In its upper part, where the reference 

 line, Sp.c, is placed, the section is almost exactly median, and shows, therefore, the 

 floor-plate or raphe of the spinal cord. In front of the cord is the vertebral artery, 

 A. vert, which joins its fellow to form the basilar artery which runs in the median 

 line the entire length of the hind-brain. The vertebral column is in the cartilagi- 

 nous stage. It is an absolutely continuous uninterrupted rod of cartilage which 

 merges at the neck with the cartilaginous skull. The entire continuous cartilaginous 

 structure is termed the chondrostyle, for the study of which comparison with the 

 neighboring sections is indispensable. Out of it both the cartilaginous skull and 

 the vertebrae are differentiated. More or less nearly in the center of the chondro- 

 style are found the remnants of the notochord, which, however, never extends 

 anterior to the pituitary body, Hyp. The division of the chondrostyle into separate 

 vertebrae is indicated by the segmental flexures" of the notochord and by the com- 

 mencing differentiation of the intervertebral ligaments. The space occupied by the 

 notochord expands in the region corresponding to the division between each two 

 vertebrae. The notochord in the intervertebral expansions is expanded and partly 

 degenerated, forming an enlarged mass of irregular strands of cells, which becomes 



