HEAD-FOLD TO TWELVE SOMITES 127 



between the somites* Near the primitive streak they disappear 

 by merging in the vascular network of the blastoderm. 



The posterior end of the endocardium divides in two branches 

 that pass out along the postero-lateral margins of the fore-gut 

 into the general vascular network of the blastoderm (Fig. 64). 

 This connection constitutes the beginning of the vitelline veins 

 through which the blood from the yolk-sac enters the posterior 

 end of the heart. 



General. The elongated form of the entire embryo and the 

 preponderance of the head are marked features of this stage. 

 The latter condition is largely due to the order of origin of parts: 

 the anterior parts preceding the more posterior in their appear- 

 ance. The head is really, therefore, in a more advanced stage 

 of development than the trunk, hence larger. The elongated 

 condition of the head and the arrangement of all its organs in 

 longitudinal sequence, however, are probably conditions of 

 phylogenetic significance, and point towards an ancestral con- 

 dition. The topographical values of the divisions of the em- 

 bryonic head are very different from those of the adult, to attain 

 which certain regions develop to a relatively enormous extent, 

 and others comparatively little. 



A number of features in the anatomy of the 12 s stage are 

 purposely omitted from this description, as they represent the 

 primordia of structures described more fully beyond; such^ for 

 instance, are the neural crest, the pronephros, etc. 



Zones of the Blastoderm. The following zones may be recog- 

 nized in the blastoderm : (1) the pellucid area surrounding the 

 embryo; (2) the vascular zone of the opaque area; (3) area vitel- 

 lina interna; (4) area vitellina externa. The pellucid area is 

 readily defined by its transparency and by the existence of the sub- 

 germinal cavity beneath it. The vascular zone is most readily 

 defined by the extension of the blood tissue which has a very 

 definite margin, coincident with the extension of the mesoblast. 

 The area vitellina includes all of the blastoderm peripheral to the 

 vascular area, and it is characterized by the presence of two 

 layers only, ectoderm and entoderm (germ-wall). It is again 

 divided into two concentric zones, internal and external. The 

 internal is much the wider (Fig. 32 A), and is characterized by 

 the existence of a perilecithal space, i.e., a slight fluid-filled 

 cavity between the entoderm and yolk continuing the subgerminal 



