THE DEVELOPMENT OF THE VASCULAR SYSTEM 283 



The origin of the lymph (plasma) itself is probably extremely complex. 

 A.t one time it was considered as the result of nitration from the blood plasma 

 through the capillary walls. If lymph originates in this way the nitration 

 is selective, for the chemical composition of the lymph differs from that of 

 the blood plasma. In all probability the lymph plasma consists of blood 

 plasma which has escaped through the vessel walls plus the products of cell 

 activity in the tissues. 



The Spleen. 



Since the spleen is generally considered as a lymphatic organ and since 

 recent researches have shown that its structure is quite comparable to that 

 of the lymph glands, it seems advisable to consider it under the head of lym- 

 phatic organs. Its ultimate origin is not yet settled and the details of its 

 later development are still obscure. The same difficulties are met with as in 

 the case of the origin and development of blood cells, for it is known that the 

 spleen plays a part in the formation of the blood cells. Its structure differs 

 from that of the lymph glands chiefly in that it possesses no distinct lym- 

 phatic sinuses; but it does possess lymph follicles (splenic corpuscles) and 

 densely cellular cords (pulp cords) which are separated by cavernous blood 

 vessels (cavernous veins). 



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

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

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

 entirely, from the mesothelium (ccelomic epithelium) of the dorsal mesogas- 

 trium. 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 elevation is produced by a local thickening and vascularization 

 of the mesenchyme, accompanied by a thickening of the mesothelium 

 which covers it; and, furthermore, 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 (mesogas- 

 trium) by a broad, thick base, but as development proceeds the attachment 

 becomes relatively smaller and finally forms only a narrow band of tissue 

 through which the blood vessels (splenic artery and vein) pass. 



Further development of the substance of the spleen consists of the break- 

 ing up of the cellular mesenchymal tissue by blood vessels and the formation 

 of the splenic corpuscles. The connective tissue trabecula, as well as the 



