THE PANCEEAS AND THE MESENTEEY. 665 



This part of the mesogaster, along the posterior border, or greater 

 curvature of the stomach, becomes produced into a double fold or 

 sac, the great omentum, which hangs down, like a curtain, over 

 the coiled mass of the intestine, close to the ventral wall of the 

 abdomen. 



Shortly after birth, the two layers of the omental sac coalesce, 

 so that the omentum becomes a single membranous layer, in 

 which fat early tends to accumulate. 



The dorsal part of the mesogaster, which is attached to the 

 dorsal body-wall, and in the thickness of which the pancreas is 

 contained, comes into close contact with the layer of mesentery 

 suspending the transverse colon, and ultimately fuses completely 

 with this ; a change which causes the pancreas to appear to lie 

 altogether dorsal to the mesentery, instead of in its substance. 



THE DEVELOPMENT OF THE CIRCULATORY 



SYSTEM. 



The general history of the development of the blood-vessels 

 in man, their relations at the different periods of embryonic and 

 of foetal life, and the changes by which at the time of birth the 

 adult circulation is established, are closely similar to those 

 already described in the rabbit. Certain differences have been 

 noticed in the mode of formation of the valves of the heart, and 

 in the development of the great veins, more especially of those 

 in relation with the liver ; these are, however, of comparatively 

 small importance, and are possibly, in some cases, due to the 

 difficulty of obtaining human embryos in satisfactory histo- 

 logical condition, and of the particular age desired. 



In the following account, which is based mainly on the 

 descriptions of Professor His, the development of the heart will 

 be dealt with first, then that of the arteries and the veins, and 

 finally a brief description will be given of the course of the cir- 

 culation in the embryo and foetus, and of the changes which 

 occur at birth. 



1. Development of the Heart. 



General Account. The early stages in the development of the 

 heart in the human embryo are known very imperfectly, and 

 only as regards the external form of the organ. 



