DEVELOPMENT OF THE CHICK FIRST PEEIOD. 



portion of the blastoderma 

 is called theareavitellina. 

 About this period also the 

 separation of the blasto- 

 derma, in the direction 

 of its thickness, becomes 

 more apparent; between 

 the serous layer, which 

 still continues limited to 

 the germinal area, and 

 the mucous layer, which 

 extends into the vitelline 

 area, there appears a 

 new lamina, which, how- 

 ever, is only distinctly 

 denned towards the pe- 

 riphery, where it ap- 

 proaches the limits of 

 the area vasculosa ; in 

 the direction of the thick- 

 ness this lamina lies in 

 the blastoderma as if it 

 belonged to both of the 

 other layers, and pene- 

 trated into their sub- 

 stance ; to distinguish 

 this less separated lami- 

 na, it is spoken of as the 

 vascular lamina, the 

 blood and blood-vessels 

 first making their ap- 

 pearance within its sub- 

 stance. This formation 

 first becomes distinctly 

 visible between the six- 

 teenth and twentieth hour 

 of incubation (fig. 329, 



297 



Fig. 324. The same vitellus, but 

 with a piece of the vitellary membrane 

 and the subjacent blastoderma re- 

 moved at a, by which the nucleus of 

 the cicatriculae, or cumulus proligerus, 

 a dark disciform substance implanted 

 in the vitellus, is brought into view. 



Fig. 325. Magnified view of the portion of the blastoderma removed 

 iu fig. 319. a, the nota, or primary streak ; b, the oblong area pellucida ; 

 c, the oval area vasculosa. 



