692 GENEEAL PHENOMENA OF EMBRYONIC DEVELOPMENT, 



is situated the medullary cavity or common ventricle of the brain and 

 spinal marrow. There is at first no distinction between the meduUary 

 or nervous structure and the containing walls : these last, including 

 the dura-matral sheath, are derived later from development of meso- 

 blastic elements. 



In birds and mammals it does not appear that there is at first any 

 line of separation or distinction between the medullary part and the 

 rest of the epiblast, but in Imirachia a difference of colour in the 

 corneous layer marks a distinction between it and the deeper part which 

 forms the medullary rudiments. 



The Uotochord. — One of the next steps in early development as 

 observed in the bird is the formation ft-om a central columnar portion 

 of the mesoblast of an axial cord occupying the future place of the 

 bodies of the vertebra and basis of the cranium. This constitutes the 

 notochord or chorda dorsaUs (see fig. 504: and sections in figs. 502 and 

 503, ch). 



dorsalis 

 into its 

 by their 



"^^ 



Fig. 502. Fig. 502.— Tkansverse Sec- 



tion THROUGH THE Ejl- 

 BRYO OF THE ChICK AND 



Blastoderm at the end 

 OP THE First Day (from 

 Kolliker). 



/;, epiljlast ; dd, lij'jioblast ; 



sp, mesoblast ; Pv, primitive 



or medullary groove ; in,. 



medullary plates ; ch, cliorda 



vwp, primordial vertebral plate ; uwh, commencement of division of mesoblast 



upper and lower lamina; ; between Rf and h the dorsal lamiuK or ridges whicb 



ajiproximation close in the medullary canal. 



(! il. 



Its structure is simply cellular, and it takes no direct part in the 

 formation of the bodies of the vertebra or cranial basis, but comes 

 later to be surrounded by the formative substance out of which these 

 parts of the skeleton are developed. In mammals and in cartilaginous 

 fishes its formation appears to be later than in birds. In man and 

 the higher vertebrates its remains are to be seen for a longer or shorter 

 period of foetal life within the cranio-vertebral osseous axis, but in the 

 lowest vertebrates, as AmfJiioxus and C ijdostomatous fishes, in which the 



Fig; 503. 



Fig. 503. — Transverse Section through the Embryo of the Chick and Ulasto- 

 DERM on the Second Day (from Kolliker). 



d d, hypoblast ; ch, chorda dorsalis ; u w, primordial vertebra; ; m r, medullary 

 plates ; h, corneous layer or epiblast ; u w h, cavity of the primordial vertebral mass ; 

 in p, mesoblast dividing at sjpinto h p I, somatopleure, and d f, splanchnopleure ; 

 IX n (/, Wolffiau duct. 



