E8TABLISHMEN1 OF THE EXTERNAL FOKM OF THE BODY. 



/"/(*/,/ tl,. r> a [> //</', /<i. The beginning of tl> ot' folding is 



juisable upon the surface of the blastoderm by meat 

 (rtain furrow.-, the innrtjiiinl <//<,,; * (( Iren/rinnrn) of His. 'I: 

 appear earlier in the anterior than in the posterior region of the 

 embryonic fundament, in OOIT8pOndeh06 with the law previously 

 enunciated, according to which 

 the anterior end of the body 

 anticipate- in development the 

 p -terior end. 



At first that part of the 

 embryonic fundament which is 

 de>tined to become the head is 

 marked oil' by means of a cres- 

 rentic groove (fig. 120). In the 

 of the Chick this is indicated 

 during the first day of incubation, 

 at a time when the first trace 

 of the nervous system becomes 

 visible. It lies immediately in 

 front of the curved anterior end 

 of the medullary ridges, with its 

 concavity directed backward. 



At a later >tage the embryonic 

 area is marked oil' laterally. In 



t he rase <:f t he embryo seen from Fig- 120.-Surface-view of the area pellucida of 

 . 101 i v a blastoderm of 18 hours, after BAI i 



the surface in tig. 121, in which In fl ,, Ilt 1)f the 1)rilll j,i VC gr0 ove (^} lies the 



medullary furrow (,//) , with the medullary 



(A). These 



out on either hide in front of the primitive 

 groove; anteriorly, on the o>ntrary, ii 

 continuous with e.ich other, and form U 

 U-hind A curved line, which represents the 



' inar.iiinal i;riMiVf. The second 

 line, 1\ iiu' i" front .if and th the 



firat, i> w' "f the amniotir fold. 



the neural tube is already j artly 

 closed and segmented into three 

 brain-v -h-les, and in which six 

 of primitive segments are- 

 laid down, there may be re- 

 i-ed at some distance from 

 these primitive segments two 

 dark Mivaks. the two lateral marginal g] 'I'lu-y 1. 



di>tinrt in passing fiom before backuanl, and wholly di.-apj-ear 

 at the end of the primitive gn . 



l-'inally. the tail end of the embryo is mark* d oil' by the p 

 marginal groove, which like the ai. 

 concavity direct* d toward the head. 



In this manner a small part of the germ layers, which alone is 

 required for the construction of the permanent body, i- aeparab d by a. 



