THE DEVELOPMENT OP THE EMBRYO ITSELF. 99 



of the first twenty-four hours such an appearance as that repre- 

 sented in Fig. 100 is presented. 



The mounds of cells forming the medullary folds are seen 

 coming in contact to form the medullary (neural) canal. 



Fig. 101.— TranBverae section of embryo chick at end of first day (after KOUilcer). M, 

 mesoblaet; H, hypoblast; m, mednllary plate; E, epiblast; m. g, medullary groove; 

 m./, medullary fold; eh, chorda dotsalis ; P, protovertebral plate; d. m, division 

 of mesoblast. 



The notochord, marking out 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- 



nf. rfLjf. 



Fig. 103. — Transverse section of chick at end of second day (EOUiker). S, epiblast; 

 H, hypoblast; e. m, external plate of mesoblast dividing (cleavage of mesoblast); 

 m.f, medullary fold; m. g, medullary groove; ao, aorta; p, pleuroperitoneal cavity; 

 P, protovertebral plate. 



age of the mesoblast, resulting in the commencement of the 

 formation of aomatopleure (body-fold) and the splanchnopleure 

 (visceral fold), is also an early and important event. These 

 give rise between them to the pleuro-peritoneal cavity. The 

 portions of mesoblast nearest the neural canal form masses (ver- 

 tebral plates) distinct from the thinner outer ones (lateral 

 plates). The vertebral plates, when distinctly marked off, as 

 represented in the figure, are termed the protovertebrce (meso- 

 blastic somites), and represent the future vertebrae and the vol- 

 untary muscles of the trunk; the former arising from the inner 

 subdivisions, and the latter from the outer (muscle-plates). It 

 will be understood that the protovertebrse are the results of 



