364 THE PHYSIOLOGY OF REPRODUCTION 



cells, with large nuclei, which he took to be wandering cells. 

 But Lenhossek proved that they existed before leucocytes or 

 lymph-cells appeared, and must therefore be formed in the viUi 

 and derived from mesoblastic cells. Hofbauer has observed them 

 also in the lumen of the foetal vessels, and suggests a possible 

 transformation to leucocytes. 



Our ideas upon the function of syncytia are largely based on 

 the investigations of His : ^ " They are not really specific tissue 

 structures, but tissue conditions requiring definite phases of 

 protoplasmic vitaUty. They occur along with a high degree of 

 activity — with increased absorption and action on material — 

 as well as with increased motility. Favourable conditions of 

 nutrition form the fundamental condition for the existence of 

 syncytia, and such conditions are certainly well offered in the 

 uterus." At the present time the syncytium is regarded as 

 of the highest importance in foetal nutrition. Strahl ^ and 

 Heinricius,^ noting its gradual and progressive diminution as 

 pregnancy advanced, supposed that it formed a part of the 

 nutriment for the embryo, but this idea has not been adopted. 

 The general theory is that it is essential in maintaining the 

 interchange of material between mother and foetus. The sub- 

 stances necessary for the building up of the foetal body may be 

 divided into two groups — diffusible and non-diffusible. The 

 passage of the former can be explained by physical laws, but it 

 is different with the non-diffusible or colloid substances. This is 

 a difficulty which does not belong to the placenta alone, but to 

 every organ of the body, and authorities are divided between 

 two theories, the mechanical and the vital. The supporters of 

 the mechanical theory hold that all the processes occurring in 

 the placenta are possible by the laws of filtration and osmosis, 

 and they have carried out numerous experiments to prove 

 that substances in solution may pass across the placenta in 

 both directions. Others, paying special attention to the nature 

 of the barrier formed by the epithelial covering of the vilh, 



' His, " Die Umschliessung der menschl. Frucht wahrend der fruhesten 

 Zeit der Schwangerschaft," Arch. f. Anat. u. Phya., anat. Abth., 1897. 



2 Strahl, "Der Bau der Hundeplacenta," Arch. f. Anat. u. Phya., anat. 

 Abth., 1890. 



' Heinrioius, " Ueber die Entwicklnng und Struktur der Placenta beim 

 Hunde,".4TO^. /. mikr. Anat., vol. xxxiii., 1889. 



