The Primative Streak. The Primative Groove 285 



cial part in the formation of certain tissues of the embryo. These 

 changes affect chiefly the embryonal area or that point in the 

 blastoderm at which the various layers are united, and the inter- 

 nal cells are massed. 



The exact method of the formation of the germinal layers is 

 not wholly free from controversy. In the embryonal area, which 

 is a circular or discoid patch, three layers of cells may be recog- 

 nized : an upper or external layer of pavement cells, the epiblast ; 

 a middle layer of larger, cuboidal cells, the mesoblast ; and a 

 lower stratum of pavement cells, the hypoblast. 



At the margin of the embryonal area, the walls of the blas- 

 todermic vesicle consist of two layers of cells representing the 

 external and internal germinal layers, while, passing slightly be- 

 yond this, the remainder of the blastodermic vesicle, constituting 

 about y^ of its total surface, consists of a single layer of cells, the 

 epiblast. 



According to Rauber and Kblliker, the uppermost layer of the 

 blastodermic vesicle, the primitive epiblast, disappears from the 

 embryonal area, to be succeeded by a new epiblast arising from 

 the mesoblast so that, according to these, the entire embryonal 

 area is ultimately derived from the inner hypoblast, which, in 

 the morula, consisted of the larger, granular, slowly multiplying 

 mass of cells. 



Late in the blastodermic stage, the embryonal area becomes 

 pyriform, its greater diameter corresponding to the long axis of 

 the blastodermic vesicle, which has now assumed the eliptical 

 form. The broader end of the embryonal area may be desig- 

 nated the anterior, or head, end and the narrower the posterior, 

 or tail, end. 



The Primitive Streak. The Primitive Groove 



As the blastodermic vesicle approaches the completion of its 

 development, there appears the primitive streak, consi.sting of an 

 axial thickening of the epiblast, at the posterior, or tail, end of 

 the embryonal area. This thickening extends longitudinally 

 and finally equals about two-thirds of the length of the area and 

 has a faint longitudinal depression known as Xh& primitive groove. 

 A cross-section of the primitive streak shows it to consist of a 

 multiplication of the deeper cells of the epiblast on the median 



