FROM LAYING TO FORMATION OF FIRST SOMITE 83 
that a gradual separation of the ectoderm takes place in the 
axis of the primitive streak beginning at the anterior end, and 
progresses posteriorly. That part in which the ectoderm is 
separated represents the head-process; it has therefore the same 
composition as the primitive streak, except that the ectoderm 
has become independent. 
Interpretation of the Primitive Streak. The discussion of the 
significance of the primitive streak involves two parts: (1) its 
morphological significance, and (2) its réle in the formation of the 
embryo. The first question involves knowledge of comparative 
embryology, which is not assumed for the purposes of this book, 
and it will therefore be considered very briefly. The fundamental 
relations of the primitive streak must define its morphological 
interpretation; the first thing to be noted is that the germ-layers, 
more especially the ectoderm and mesoderm, are fused in the 
primitive streak; second, the differentiated part of the embryo 
is formed in front of it; third, the neurenteric canal occupies the 
anterior end of the primitive streak; fourth, the anus forms at its 
posterior end. Now these characters are exactly those of the 
blastopore or primitive mouth of lower vertebrates, that is of the 
aperture of invagination of the archenteron. For these reasons, 
and because in all other essential respects the primitive streak 
corresponds to the blastopore, it must be interpreted as the homo- 
logue of the latter. It is to be regarded, therefore, as an elongated 
blastopore, and the primitive groove as a rudimentary archenteric 
invagination. 
This interpretation raises the question as to its relation to 
the original marginal area of invagination of the entoderm. Can 
these two things be really different stages of the same thing? 
The concrescence theory gives a theoretical basis for their iden- 
tification. It will be remembered that the margin of invagina- 
tion represents a small section of the margin of the primitive 
blastoderm in the pigeon, and, by inference, in the chick also. 
The remainder of the margin where the zone of junction persists 
is the margin of overgrowth. Now we assume that the closure 
of the original marginal area of invagination proceeds by con- 
crescence or coalescence of its lips, beginning in the middle line 
behind, thus producing a suture which is the beginning of the 
primitive streak. Let the above circles (Fig. 48) represent the 
blastoderm in four stages of closure of the original area of invag- 
