350 OUTLINES OF CHORDATE DEVELOPMENT 



ducts and the blood-vessels represent the primarily formative 

 outgrowths of the ducts from the vascular endothelium, from 

 which the cords have extended and fused secondarily, but 

 direct observations to this effect are wanting.) 



The anterior lymphatic hearts apparently disappear early. 

 The posterior lymph hearts attain their maximum development 

 during the fourteenth and fifteenth days, when they begin to 

 retrogress and disappear entirely ten to fourteen days after 

 hatching. 



The spleen arises, during the fourth day, from a proliferation 

 of peritoneal cells in the base of the mesentery just above the 

 pancreatic region. It enlarges rapidly through continued cell 

 proliferation and the accumulation of a mesenchymatous 

 stroma. Its spaces, without definite endothelial walls, are 

 directly continuous with the sinusoidal origins of its efferent 

 vein, and from these spaces splenic cells enter the blood stream 

 and become converted into blood corpuscles. 



V. THE CAVITIES OF THE BODY 



The folding-off of the embryo from the yolk completes the 

 roughing-in of the body cavity. From the very beginning the 

 general embryonic body cavity shows signs of the differentiation 

 of the region surrounding the heart as the pericardial cavity. 

 We have already traced the origin of this part of the ccelom, in 

 describing the origin and formation of the heart. We have 

 also seen how the body cavity proper is formed and closed, and 

 how it is partially divided longitudinally by the dorsal mesen- 

 tery. It remains now for us to consider the essential steps 

 in the complete separation of the pericardial cavity and the 

 further subdivision of the primary body cavity. 



Throughout the early stages of development the pericardial 

 cavity is only incompletely closed off from the body cavity, 

 since it is only partly closed posteriorly by the mesoderm in 

 the wall of the anterior intestinal portal, and the vessels which 

 are entering the heart from the yolk-sac. The formation of 

 the lateral mesocardia (see above) extends the separation of the, 



