DEVELOPMENT OF THE CHICK FIRST PERIOD. 295 



period anterior to the commencement of incubation, the ger- 

 minal vesicle having burst ; the upper disciform layers of the 

 germ and germinal cumulus only separate more and more. 

 After the egg is thus perfected, it is forced rapidly through 

 the cloaca. In other birds, it is here perhaps that the egg 

 receives, in part at least, the beautiful colours, red, green, yel- 

 low, brown, &c., in various shades, which are so frequently 

 met with, and which appear to be so many tints of the colour- 

 ing matter of the blood chemically altered. 



EARLIEST PERIOD IN THE DEVELOPMENT OF THE CHICK, 

 FROM THE FIRST APPEARANCE OF THE EMBRYO TO THE 

 FIRST TRACES OF CIRCULATION. 



[ 482. The first period in the development comprehends 

 about two days. In the first hours of incubation, the germ 

 separates itself more from the vitellus and vitellary membrane, 

 to which, however, it still continues in some sort attached ; 

 the germ acquires more of a membranous consistence, and 

 the space between it and the germinal cumulus, which is filled 

 with fluid, becomes somewhat larger. Towards the sixth, or 

 between that and the eighth hour, a parting or resolution in 

 the now foliaceous germinal membrane, which proceeds from 

 the centre towards the periphery, is apparent ; a clear rounded 

 space, about a line in diameter, is produced in the middle, 

 this is the area pellucida s. germinativa the pellucid or ger- 

 minal area (fig. 319, e) ; the germinal membrane at the same 

 time becomes darker in the circumference, and surrounds the 

 transparent peUucid area like a ring, which is also about a line 

 in breadth (fig. 319, d) ; this is the future area vasculosa, or 

 vascular area. The cumulus proligerus is seen in the deeper 

 parts shining through the centre of the germinal membrane. 

 At this time two or three annular lines appear drawn around 

 the circumference of the germinal membrane the halones 

 (fig. 319, c, c] ; these are circular ridges or walls formed in 

 the vitellus, between which there are furrows filled with thin- 

 ner fluid. Now also the germinal membrane may be observed 

 to show a disposition to separate into two layers, which are, 

 indeed, still intimately connected, but even at this early period 

 are in point of structure different. They are always particu- 

 larized as the lamina of the germinal membrane, the superior 



