i8 



involution at the tail end forms the arms, this involution 

 grows back, and meets the blind termination of the hind 

 gut, and the two merge together, and the clonca is com- 

 pleted. 



The main artery from the heart dilates at its com- 

 mencement, and forms the bulbus artenosus as early as the 

 third day, it now becomes divided by a partition into two 

 arteries, one the pulmonary artery, (going to the lungs), and 

 the aorta. The heart which we saw had become bilocular 

 is further developed, and each cavity is divided by a partition, 

 thus four chambers are formed. 



All the nerves of the body are developed from the 

 mesoblast. 



The bones of the head now appear, and on the sixth day, 

 the beak appears as a minute white speck on the tip of the 

 nose. All these changes advance gradually, and about the 

 twentieth day, the beak of the chick is thrust through into 

 the air chamber situated at the broad end of the egg, 

 respiration then commences, and the lungs become further 

 developed. 



Although the air in the air chamber is constantly 

 removed through the porous egg shell, in a very short time 

 it becomes insufficient to supply the wants of the chick. 

 The besion de rcspirer causes the chick to peck with its beak 

 against the shell, and to break it, and it then steps out into 

 the world. 



NOTE. — This paper was illustrated by a large number of living 

 and dead embryos in different stages of development. By placing 

 the embryo on a " hot stage," the pulsations of the heart were 

 shown under the microscope. The microscopical sections pre- 

 pared by the author were magnified, and thrown on to a screen, 

 by means of a micro-photographic apparatus and the electric light, 

 very kindly lent by and manipulated by Mr. VVilliam Webster, jun. 



