DEVELOPMENT OF THE CHICK PIEST PERIOD. 305 



to the middle of the second. The area vasculosa (figs. 330 

 and 333, c) has enlarged, and from a form rather elongated, 

 has assumed one that is rounder. Its outer circumference is 

 beset with darker aggregated-looking masses (fig. 333) ; sin- 

 gle isolated points appear, and between these clefts are formed, 

 that by and by run together and form channels, which unite in 

 meshes with one another ; in these channels a clear colourless 

 or extremely pale yellow fluid can by and by be distinguished 

 in motion this is the blood. The halones (fig. 330), which 

 had become more sinuous towards the beginning of the second 

 day, now vanish entirely. Along with these occurrences in 

 the periphery of the vascular lamina, the development of the 

 heart has been advancing in the centre, under the transparent 

 germinal area and the serous layer of the embryo. The vas- 

 cular lamina becomes thicker, and appears darker in this 

 point ; the heart shows itself as a somewhat sinuous sac, in- 

 terposed between and pushing apart the mucous and serous 

 laminae (fig. 335, B, eP). As the development advances, the 

 heart is observed from the under or abdominal aspect of the 

 embryo as a sac, simple and undefined anteriorly, of greater 



a 



Fig. 336. An incubated vitellus of the jackdaw's egg ; A, of the na- 

 tural size ; B, magnified a, vitellary membrane ; b, b, b, halones ; c, 

 embryo ; d, area pellucida , e, area vasculosa. (^Compare with figs. 330 

 and 333.) 



