FCETAL NUTRITION: THE PLACENTA 361 



attention of few morphologists, among whom Turner, the 

 " grand-master of placental research '' (Hubrecht ^), was facile 

 princeps. But within recent years investigations have been 

 carried out on many orders of placental mammals. Of these 

 the most important are the researches of Duval and Hubrecht, 

 which have estabhshed that the discoid placenta is essentially 

 " a maternal haemorrhage encysted by foetal elements." 



II. Structure and Functions of the Epithelial 

 Investment of the Villi 



The cellular layer of the vilU is a temporary structure, and 

 disappears to a great extent comparatively early in pregnancy. 

 It is generally looked on as the mother zone of the outer syncytial 

 layer. Strahl ^ states, however, that in one of the new-world 

 apes it is not present at a stage as early as that of Peters' human 

 ovum, though a thick syncytial layer is present. Processes of it 

 precede the mesoblastic outgrowths in the formation of the vilh, 

 and by a special proliferation of the cells at the tips of the villi, 

 the " Zellsaulen " of Langhans, an attachment to the decidua 

 is effected. While present, the cellular layer hes in the path 

 by which the nutriment for the foetus is carried to the villous 

 capillaries, but it is not known whether it exerts any metabohc 

 influence. Peters has suggested, without any very definite 

 evidence, that it may have a coagulating action on maternal 

 blood, necessitating the interposition of the syncytial layer. 



The syncytium is more permanent. In the earhest human 

 ovum yet examined it already constitutes a considerable mass, 

 and a similar thickening over the whole or part of the circum- 

 ference of the blastocyst occurs early in all the Deciduata. 

 Where a decidua reflexa exists, the early proHferation appears to 

 be related to the excavation of the cavity in which the ovum hes. 

 In discoid placentae the mass is vacuolated, and maternal blood 

 is contained in the lacunae. In the later stages of pregnancy it 

 forms an attenuated membrane over the vilh, and may wholly 



' Hubrecht, "The Placentation of Erinaceua europcBua," Qiiar. Jour. 

 Micr. Sci., vol. xxx., 1889. 



^ Strahl, "Ueber Plaoentarsyncytien," Anat. Anz., vol. xxix., Erganz- 

 ungsoh., 1906. 



