AVES. 



127 



tioned which is a linear body placed in the posterior region of the 

 blastoderm. This body, the nature of which will be more fully ex- 

 plained in the chapter on the comparative 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. 52) ; although 

 there is no ontogenetic process in the Amniota, like the concrescence 

 in Elasmobranchii. 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 succeed- 

 ing changes we may at first confine 

 our attention to the area pellucida. 

 As this gradually assumes an oval 

 form the posterior opacity becomes re- 

 placed 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. In the region 

 in front of it the blastoderm is still 

 formed of two layers only, but in the 

 region of the streak itself the structure 

 of the blastoderm is greatly altered. 

 The most important features in it are 



represented in fig. 95. This figure shews that the median portion 

 of the blastoderm has become very much thickened (thus producing 

 the opacity of the primitive streak), and that this thickening is caused 



JJT.S 



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. 



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



IN FIG. 9-4, 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. 



by a proliferation of rounded cells from the epiblast. In the very 

 young primitive streak, of which fig. 95 is a section, the rounded 

 cells are still continuous throughout with the epiblast, but they form 

 nevertheless the rudiment of the greater part of a sheet of rneso- 

 blast, which will soon arise in this region. 



