THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



283 



For some time the spleen was considered as a derivative primarily of the 

 mesenchyme in the region of the dorsal mesogastrium. More recently, how- 

 ever, investigators have taken the view that it arises partly, or possibly entirely, 

 from the mesothelium (ccelomic epithelium) of the dorsal mesogastrium. In 

 human embryos during the fifth week the anlage of the spleen appears as an 

 elevation on the left (dorsal) side of the mesogastrium (Fig. 259). This eleva- 

 tion is produced by a local thickening and vascularization of the mesenchyme, 



Aorta 



Omental 

 bursa 



Mesonephros 



Spleen 



Dorsal 



mesogastrium 

 (greater omentum) 



Abdominal cavity 

 (ccelom) 



Stomach 



Bile duct 



Ventral mesogastrium 

 (lesser omentum) 



FIG. 259. From transverse section through stomach region of a 14 mm. 

 pig embryo. Photograph. 



accompanied by a thickening of the mesothelium which covers it; and, further- 

 more, the mesothelium is not so distinctly marked off from the mesenchyme as 

 in other regions. Cells from the mesothelium then migrate into the subjacent 

 mesenchyme and the latter becomes much more cellular (Fig. 260). The 

 migration is brief, and in embryos of about forty-two days has ceased, and the 

 mesothelium is again reduced to a single layer of cells. The elevation becomes 

 larger and projects into the body cavity. At first it is attached to the mesentery 

 (mesogastrium) by a broad, thick base, but as development proceeds the at- 



