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 role 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 w^here 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. 43) represent the 

 blastoderm in four stages of closure of the original area of invag- 



