CH. LIX.] THE FOZTAL CIRCULATION 891 



occur in muscles, glands, tendons, and especially in serous membranes. 

 On account of their affinity for pyrrhol-blue they were originally 

 termed pyrrhol cells, and it seems probable that they originate in 

 the bone marrow. 



By means of such intra-vitam stains one can further differentiate 

 the Kupffer-cell of the liver, the reticulum cell of lymph glands and 

 spleen, the interstitial cell of the testis, the follicular cell in the 

 maturing follicles of the ovary, the cortical cell of the suprarenal, 

 the epithelial covering of the choroid plexuses, and the cells which 

 line the convoluted tubules of the kidney. 



When pregnancy occurs in the stained animal, the appearance 

 and behaviour of the placenta are most striking; the blue colour 

 disappears from the skin and is concentrated in the uterus, and 

 in time the latter, forming a centre of attraction for the dye, 

 ultimately dispossesses all the remaining tissues of their blue. In 

 the uterus it is the free cells of the decidua basalis where the stain 

 is mainly found. In quite early stages the stained cells penetrate 

 into the primitive placenta and cast off their stained granules, 

 which are snatched up by foetal cells in the way nutritive material 

 is. But when once the placenta has attained maturity, the dye 

 is found only in the foetal cells which form the layer which separates 

 the maternal and foetal tissues. The foetus itself remains per- 

 fectly colourless, the stain not being able to penetrate this protective 

 barrier. Further research has shown another important point, 

 for the same cells which vigorously absorb the vital stain store also 

 glycogen, fat, and haemoglobin temporarily before these substances 

 pass into the foetal circulation. The avidity of such cells for the 

 dye is thus connected with their functional activity in relation to 

 really nutritive material; the importance of vital staining in 

 embryological research is therefore apparent. 



Equally important are its applications to pathological research, 

 but into this aspect of the question it will be beyond our purpose 

 to pass. 



THE FCETAL CIRCULATION 



We shall not enter into the complex manner in which the heart 

 and blood-vessels of the foetus develop from the embryonic rudiments ; 

 but when these are fully formed the circulation of the blood is found 

 to differ considerably from that which occurs after birth. It will 

 be convenient to begin its description by tracing the course of the 

 blood, which, after being carried to the placenta by the two umbilical 

 arteries, has returned, oxygenated and replenished, to the foetus by 

 the umbilical vein. 



It is at first conveyed to the under surface of the liver, and there 

 the stream is divided, a part of the blood passing straight on to the 



