THE EARLY DEVELOPMENT OF THE CHICK 257 



all the structures so far described, only those in the vicinity 

 of the head process, represent actual embryonic rudiments; 

 these are, that thickening of the ectoderm described as the 

 medullary (neural) plate, and the axial endoderm and meso- 

 derm lying directly beneath it. The substance of the primi- 

 tive streak, therefore, lies posterior to the embryo proper, and 

 the boundary between the two is indicated by the position of 

 the primitive pit; 



As the embryo becomes more definitely established, it elon- 

 gates posteriorly, while the primitive streak correspondingly 

 shortens. The relation between the two structures is such 

 that the embryo draws its substance from the anterior end of 

 the primitive streak, or, we may say, the materials of the 

 anterior end of the primitive streak are slowly redifferentiated 

 and payed into the hinder end of the embryo. The region 

 where this process of redifferentiation is proceeding is indicated 

 as Hensen's node (primitive knot). Finally the primitive 

 streak is wholly transformed into embryonic structures, but 

 this is not fully accomplished until a relatively late stage in 

 development (second day of incubation). To the question as 

 to how much of the definitive embryo is formed of structures 

 primarily anterior to the primitive streak, no precise answer 

 can be given. In a general way, however, it may be said that 

 the region of the primary location of the primitive pit corre- 

 sponds roughly with the boundary between hind- and mid- 

 brain, and that structures developing anteriorly to this level 

 were related originally to the region of the head process, rather 

 than to the primitive streak. 



We are now prepared to understand the essential homologies 

 of the primitive streak, having learned of its relation to the 

 embryo, on one hand, and to the structures of the gastrula, on 

 the other. Theoretically the primitive streak is to be inter- 

 preted as the result of a modified process of concrescence (con- 

 fluence). The chief alterations of the typical process of con- 

 crescence (confluence) are apparently due to the relatively 

 enormous mass of yolk, and the consequent modification of 

 the gastrula into a flat sheet, more or less intimately related to 



