94 ANIMAL PHYSIOLOGY, 
of the first twenty-four hours such an appearance as that repre- 
sented in Fig. 104 is presented. 
mf 
E 
M. GON 
5 E 
H No Cee = 
seed = — — 
©) 3} sHo 
Fie. 104.—Transverse section through the medullary groove and half the blastoderm of a 
chick of eighteen hours (Foster and Balfour). £, epiblast ; M, mesoblast ; H, hypoblast ; 
mf, medullary fold ; mg. medullary groove ; ch, notochord. 
The mounds of cells forming the medullary folds are seen 
coming in contact to form the medullary (neural) canal. 
Fic. 105.—Transverse section of embryo chick at end of first day (after Kélliker). M, meso- 
blast ; H, hypoblast ; m, medullary plate; EZ, epiblast : mg, medullary groove ; mf, me- 
dullary fold ; ch, chorda dorsalis ; P, protovertebral plate ; dm, division of mesoblast, 
The notochord, marking ott the future bony axis of the 
body, may also be seen during the first day as a well-marked 
linear extension, just beneath the medullary groove. The cleav- 
Fic. 106.—Transverse section of chick at end of second day (Kélliker). £, epiblast: H. hypo- 
blast ; e. m, external plate of mesoblast dividing tcloavare of mesoblast) ; m. f, medullary 
told m.g, medullary groove ; ao, aorta ; p, pleuroperitoneal cavity ; P, protovertebral 
plate. 
age of the mesoblast, resulting in the commencement of the 
formation of somatopleure (body-fold) and the splanchnoplewre 
(visceral fold),is also an early andimportant event. These give 
rise between them to the plewro-peritoneal cavity. The portions 
of mesoblast nearest the neural canal form masses (vertebral 
plates) distinct from the thinner outer ones (lateral plates). 
