395 THE PHYSIOLOGY OF REPRODUCTION 



by Weber and several of his successors ; Winkler l proved it to be 

 a constant phenomenon, and gave it the name " Schlussplatte " ; but 

 it was Langhans who first described its relations, and suggested its 

 probable origin from the foetal epiblast. The cellular layer, according 

 to Langhans, was mesoblastic. Kastschenko 2 first described both 

 layers as epiblastic, and showed that the outer layer was a syncytium 

 or mass of nucleated protoplasm without cell-boundaries. Such 

 investigations led to the feeling that the structure of the placenta 

 could only be understood by tracing its development from very early 

 periods of gestation. Hence the search for and examination of young 

 human ova were stimulated, and the study of the uterine condition 

 in age-series of pregnant animals was begun. Up to this time the 

 chief controversies had raged around the human placenta. Com- 

 parative placentation had engaged the attention of few morphologists, 

 among whom Turner, the "grand-master of placental research" 

 (Hubrecht 3 ), was facile princeps. But within recent years investi- 

 gations have been carried out on many orders of placental Mammals. 

 Of these the most important are the researches of Duval and 

 Hubrecht, which have established that the discoid placenta is 

 essentially " a maternal haemorrhage encysted by foetal elements." 



II. STRUCTUEE AND FUNCTIONS OF THE EPITHELIAL 

 INVESTMENT OF THE VILLI 



The cellular layer of the villi 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 4 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 villi, 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 lies 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 metabolic 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. 



1 Winkler (F. N.), "Zur Kenntnis der menschlichen Placenta," Arch, f, 

 Gyniik., vol. iv., 1872. 



2 Kastschenko, " Das menschliche Chorionepithel und dessen Rolle bei der 

 Histogenese der Placenta," Arch. J. Anat. u. Phys., Anat. Abth., 1885. 



3 Hubrecht, "The Placentation of Erinaceus europceus" Quar. Jour. Micr. 

 Science, vol. xxx., 1889. 



4 Strahl, "Ueber Placentarsyncytien," Anat. Anz., vol. xxix., Erganzungsh., 

 1906. 



