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 12s 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, ete. 
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, 7.e., a slight fluid-filled 
‘avity between the entoderm and yolk continuing the subgerminal 
