284 



TEXT-BOOK OF EMBRYOLOGY. 



tachment 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 breaking 

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

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

 the spleen are derived from the original mesenchymal tissue. The blood 

 vessels become dilated in parts of their course to form the cavernous vessels 

 (cavernous veins) which are separated by the pulp cords. The connective 

 (reticular) tissue of the pulp cords is a derivative of the mesenchyme, but the 

 origin of the various types of cells in the cords is not certain. The adventitia of 



Mesothelium Anlage of spleen 



Migrating cells 



Mesenchyme 



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FIG. 260. From section through dorsal mesogastrium (anlage of spleen) of a chick embryo 

 of 3 days and 21 hours incubation. Tonkoff. 



the walls of some of the small arteries becomes infiltrated with lymphocytes to 

 form the splenic corpuscles (lymph follicles) . 



It has already been mentioned that during foetal life the spleen is a hemato- 

 poietic organ, that is, both leucocytes and nucleated red blood cells proliferate 

 within it. Normally, the formation of erythrocytes stops at or soon after 

 birth. In severe anaemia or in pernicious anaemia in postnatal life, however, 

 the presence of dividing nucleated red blood cells suggests a return to 

 embryonic conditions. Still the question arises as to the origin of these nucle- 

 ated forms (erythroblasts) . It has also been suggested that the spleen acts as a 

 destroyer of worn-out erythrocytes, for in many cases apparent remnants of 

 the latter have been observed within the cytoplasm of the "spleen cells." The 

 lymphocytes proliferate to a certain extent in the splenic corpuscles, and in that 

 way, at least, the spleen serves as a base of supply for leucocytes. There is a 



