THE SPIXAL MARROW. 747 



Fi"-. 545. — Embkyo of the Dog more advanced, seen from above (after Bisclioff ). 



The medullaiy canal is now closed in ; c, the anterior encephalic vesicle ; o, the primi- 

 tive optic vesicle in communication with the anterior encephalic ; au, the primitive 

 auditory vesicle opposite the third encephalic vesicle ; am, the cephalic fold of the amnion 

 enclosing the anterior third of the embryo ; ov, the omphalo-mesenteric vein entering the 

 heart posteriorly ; in-, the proto- vertebral divisions, now become numerous. 



nervous centre is formed more immediately from the thickened medul- 

 lary plates of the involuted epiblast, the ridges of which, rising from 

 the surface of the blastoderm, become united dorsally along the middle 

 line into a hollow medullary tube of a cylindrical form. This tube 

 becomes dilated at its anterior or cephalic extremity, and this dilated 

 portion becomes divided by two partial constrictions into the three 

 primary cerebral or encephalic vesicles, which, as representing funda- 

 mentaf portions of the brain, have been termed the fore-brain, mid-brain 

 and hind-brain. The spinal portion retains its more uniform cylindrical 

 shape, escepting towards the caudal extremity, where it is longer of 



Fi<r. ZiPj. 



Fig. 546. — Transverse Sec- 

 tion THROUGH THE Ejl- 

 BRYO OF THE ChICK, AND 



■^^A, Blastoderm at the end 



OP THE First Day. Mag- 

 nified FROM 90 TO 100 

 TIMES (from Kolliker). 



d'/> tUuh d d i,a-p c/i /i, epiblast ; (?c?, hypoblast ; 



sp, mesoblast ; Pv, medullary 



groove ; m, medullary plates ; 



c7i, chorda dorsalis ; utrp, proto-vertebral plate ; uw7i, commencement of di^'ision of 



mesoblast into its upper and lower laminte ; between i?/ and h the dorsal laminte or 



ridges which by their approximation close in the medullary canal. 



closing, and forms for a time a flat open rhomboidal dilatation. The 

 continuous cavity enclosed within the primitive medullary tube is the 

 same with that which, variously modified, afterwards constitutes the 

 central ventricles of the brain and canal of the spinal cord. 



The formative cells composing the medullary substance are at first 

 spherical, but they afterwards become elongated and spindle-shaped, 

 and increase rapidly by multiplication. They represent at first the 



Fig. 547. 



/-r 





Fig. 547. — Transverse Section of the Embryo Chick on the Second Day. Mag- 

 nified FROM 90 to 100 times (from Kolliker). 



The explanation of the letters is in part the same as in the foregoing figure, mr, the 

 medullary tube now closed along the dorsal line and covered in by continuous epibkist ; 

 nwh, hollow of the proto-vertebral mass ; mp, mesoblast external to the protovertebroe 

 dividing into hpl, somatopleure, and df, splanchnopleure ; ao, one of the primitive 

 aortas ; iinff, intermediate mass connected with the origin of the Wolffian body. 



