1034 



EMBRYOLOGY. 



The Diiddle cerebral vesicle gives origin to the cerebral crura, the corpora 

 qiiiidrigemina, and the aqueduct of Sylvius or middle ventricle. It deserves to 

 be named the vesicle of the corpora quadrigemiua of the middle brain. 



The third vesicle becomes doubled, forming the cerebellar vesicle, or iwHterior 

 train, from which arises the cerebellum, and the vesicle of the medulla oblongata 

 which originates that body. 



The middle vesicle is that which, at first, increases most rapidly in volume ; 



it soon, however, ceases to grow, in 

 Fig. 568. order to allow the anterior cell to 



become developed, when the brain 

 assumes its ovoid form, with a pre- 

 dominance of the anterior part. 



Towards the end of the first 

 third of intra-uteriue life, nearly all 

 the parts of the brain are distinct : 

 the two hemispheres have become 

 isolated by the development of the 

 septum lucidum ; the convolutions 

 appear on their surface ; and the cor- 

 pora quadrigemiua and cerebral cura 

 are distinctly defined. It is not until 

 a little later that the cerebellum is 

 distinguishable, as well as the pons 

 Varolii, medulla oblongata, corpora 

 restif ormia, and corpora pyramidalia. 

 8. Dpvelojmient of the spinal cord. 

 — The neural canal is the first trace 

 of the spinal cord ; it occupies the 

 whole length of the vertebral spine, 

 and its cavity communicates, in 

 front, with the fourth ventricle. 

 When the spine is developed, the 

 cord does not increase proportion- 

 ately in length, and appears to ascend 

 in the vertebral canal ; it stops about 

 the middle of the sacrum in the 

 equine foetus, but ascends higher 

 in the other species. During this 

 apparently ascensional movement is 

 developed the filum terminate, and 

 the nerves of the cauda eqimia. 



The walls of the neural canal, at first very thin, increase in thickness by the 

 appearance of the nerve of the cord. Soon they divide into two layers — an in- 

 ternal, the epithelium of the central canal ; the other external — the grey substance 

 of the cord. Gradually the canal contracts, and the marrow is seen, with its 

 longitudinal furrows. 



At the end of the first month, the inferior roots are in existence, as well as 



the spinal ganglia ; the latter are developed at the expense of the protovertebrae, 



as will be shown presently. The superior roots are formed some time afterwards. 



The envelopes of the cerebro-spinal centres are furnished by the protoverte- 



EMBRYO OF THE CHICK AT FROM THIRTY TO 

 THIRTV-SIX HOURS. 



T.a, Anterior braiu; m.h, middle brain; h.h, posterior 

 l)rain ; op.v, optic vesiclt- ; du.p, olfactory fossa ; 

 o.f, vitelline vein; p.v, protovertebras, or meso- 

 blastic somite ; m.f, line of junction of the 

 medullary laminae above the neuraxis ; s.r, 

 rhomboidal sinus ; t, caudal fold ; p.r, remains of 

 the primitive groove ; a.p, area pellucida. 



