ESTABLISHMENT OF THE EXTERNAL FORM OF THE BODY. 



199 



from the area pellucida. The beginning of the process of folding is 

 recognisable upon the surface of the blastoderm by means of 

 certain furrows, the marginal grooves (Grenzrinnen) of His. These 

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

 embryonic fundament, in correspondence with the law previously 

 enunciated, according to which 

 the anterior end of the body 

 anticipates in development the 

 posterior end. 



At first that part of the 

 embryonic fundament which is 

 destined to become the head is 

 marked off by means of a cres- 

 centic groove (fig. 120). In the 

 case 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 stage the embryonic 

 area is marked off laterally. In 



the case of the embryo Seen from Fig. 120._ Surface-view of the area pellucida of 

 . . , t , n . ,., a blastoderm of 18 hours, after BALFOUR. 



the surface m fig. 121, in which Iu frout , of the priniiti ve groove ( pr) lies the 

 the neural tube is already partly 

 closed and segmented into three 

 brain-vesicles, and in which six 

 pairs of primitive segments are 

 laid down, there may be re- 

 cognised at some distance from 

 these primitive segments two 



dark streaks, the two lateral marginal grooves. They become 

 less distinct in passing from before backward, and wholly disappear 

 at the end of the primitive groove. 



Finally, the tail-end of the embryo is marked off by the posterior 

 marginal groove, which like the anterior is crescentic, but has its 

 concavity directed tow r ard the head. 



i'n this manner a small part of the germ-layers^ which alone is 

 required for the construction of the permanent body, is separated by a 



medullary furrow (me), with the medullary 

 ridges (A). These diverge behind and fade 

 out on either side in front of the primitive 

 groove ; anteriorly, on the contrary, they are 

 continuous with each other, and form n arch 

 behind a curved line, which represents the 

 anterior marginal groove. The secoud curved 

 line, lying in front of and concentric with the 

 first, is the beginning of the atuniotic fold. 



