AVES. 



153 



of the blastoderm, but are placed near its centre. Behind the 

 embryo there is however a peculiar structure the primitive 

 streak above mentioned which is a linear body placed in the 

 posterior region of the blastoderm. This body, the nature of 

 which will be more fully explained in the chapter on the com- 

 parative development of Vertebrates, is really a rudimentary 

 part of the blastopore, of the same nature as the linear streak 

 behind the embryo in Elasmobranchii formed by the concrescence 

 of the edges of the blastoderm (vide p. 64) ; although there is no 

 ontogenetic process in the Amniota, 

 like the concrescence in Elasmo- 

 branchii. The relations of the 

 blastopore in Elasmobranchii and 

 Aves is shewn in figs. B and C of 

 the diagram (fig. 93). 



In describing in detail the suc- 

 ceeding changes we may at first 

 confine our attention to the area 

 pellucida. As this gradually as- 

 sumes an oval form the posterior 

 opacity becomes replaced by a very 

 dark median streak, which extends 

 forwards some distance from the 

 posterior border of the area (fig. 

 94). This is the first rudiment of the primitive streak. 



FIG. 94. AREA PELLUCIDA OF 

 A VERY YOUNG BLASTODERM OF 

 A CHICK, SHEWING THE PRIMITIVE 

 STREAK AT ITS FIRST APPEARANCE. 



pr.s. primitive streak ; ap. area 

 pellucida ; a.op. area opaca. 



In the 



FIG. 95. TRANSVERSE SECTION THROUGH A BLASTODERM OF ABOUT THE AGE 

 REPRESENTED IN FIG. 94, SHEWING THE FIRST DIFFERENTIATION OF THE PRIMITIVE 



STREAK. 



The section passes through about the middle of the primitive streak, pvs. primitive 

 streak; ep. epiblast; hy. hypoblast; yk. yolk of the germinal wall. 



region in front of it the blastoderm is still formed of two layers 



