EXPLANATION OF THE PLATES. 



PLATE X. 



DEVELOPMENT OF THE EMBRYO OF THE CHICK. 



Fig. 1. The appearance of the cicatricula in the yolk prior 

 to incubation. 



Fig. 2 The same at the end of the first day of incubation ; 

 the halones are now distinctly visible, as also the 

 area pellucida, and nota primativa, or first rudiment 

 of the young chick. 



Fig. 3. The same at the termination of the thirty-sixth hour 

 of incubation ; the halones have become more marked 

 and expanded, the nota primitiva larger, and traces 

 of blood-vessels are now for the first time dis- 

 tinctly visible in the germinal membrane. 



Fig. 4. The same at the close of the second day ; the pulsa- 

 tion of the heart and the vessels of the area vasculosa 

 are clearly visible ; within them the coloured cor- 

 puscles may be seen circulating. 



Fig. 5. The same at end of the third day of development ; 

 the area vasculosa has now extended itself to two 

 or three times its former dimensions. 



Fig. 6. The embryo on the conclusion of the fourth day ; the 

 head, the eye, and the budding of the allantois 

 are now seen in addition to the parts previously 

 noticed. 



Fig. 7. The embryo at the termination of the fifth day ; the 

 wing and the foot have made their appearance; 

 the limits of the area vasculosa cannot now be 

 seen, it extending over two thirds of the surface of 

 the egg ; after this and the following day, the 

 periods of its complete development, the area 

 suffers an arrest of growth, and the vessels contract 

 and carry but little blood, until at length they are 

 entirely obliterated. The allantois has on this day 

 attained a considerable size, and its further growth 

 proceeds with the utmost rapidity. 



