266 STUDY OF PIG EMBRYOS. 



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 differentiation 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 sub- 

 divided 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. In front of the choroid plexus the arching 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 cartilaginous 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. Out of it both the cartilaginous skull and the verte- 

 bra are differentiated. More or less nearly in the center of the chondrostyle are 

 found the remnants of the notochord, which, however, never extends anterior to 

 the pituitary body, Hyp. The division of the chondrostyle into separate verte- 

 brae is indicated by the modifications of the notochord and by the commencing 

 differentiation of the intervertebral ligaments. The space occupied by the noto- 

 chord expands in the region corresponding to the division between each two ver- 

 tebrae. The notochord in the intervertebral expansions is partly degenerated, 

 forming an enlarged mass of irregular strands of cells. From each such mass 

 goes off a narrow extension of the notochord, through what is to become the body 

 of the vertebra. Sometimes this extension is continuous with the intervertebral 

 portions of the notochord, but more usually it forms a series of isolated fragments, 

 for the notochord in the parts corresponding to the bodies of the vertebrae is 

 already in process of resorption. The diameter of the chondrostyle is nearly uni- 

 form in the vertebral region, but is a little smaller in the part corresponding to 

 each body of a vertebra and a little wider in the parts corresponding to the inter- 



