2-38 Professor Clark on a Case 



in a vessel which bends downwards and inwards to join the 

 heart. 



The formation of the heart is absolutely similar to that of 

 these vessels of the areola, and takes place before the fluids in the 

 latter can be seen to move. By the bending- of the ends of the 

 dorsal lamina?, the blastoderma is drawn down at an angle from 

 its general direction upon the yolk, and forms a fold under the 

 head of the embryo, in which fold its serous and mucous portions 

 are considerably separated. This is a most important movement, 

 for a triangular cavity is thus left between them, which is soon 

 occupied by a collection of globules from the vascular intervening 

 membrane : and it is from this large collection of globules that 

 the heart is formed, by a separation of their component parts. 

 They become, in the same way as the vessels of the areola, a 

 longitudinal sac, with two posterior angles, containing a fluid 

 mass. The angles receive the descending vessels from the ter- 

 minal vein of the areola. At the very time that the heart is thus 

 formed, the fluid contents of the canal for the spinal cord begin 

 to assume a degree of solidity. The brain now consists of three 

 vesicles containing a fluid, which are the rudiments of the me- 

 dulla oblongata, of the corpora quadrigemina, and of the cerebrum. 

 They lie, in the order mentioned, in a curved line continued from 

 the spinal cord : and, with other parts afterwards superadded, 

 may be observed before any vestige of a blood vessel can be de- 

 tected near them. Thus the formations of the heart, and of the 

 brain are synchronous, and have no connection as cause and effect. 



It has been said that as the dorsal laminae are bent the ger- 

 minative membrane forms an angle, where it is drawn down by 

 the head from the general direction of that membrane. This 

 operation does not occur at the head alone, the same takes place 



