414 THE PHYSIOLOGY OF REPRODUCTION 



is then transformed into a syncytium and invests the villi 

 externally. Heinricius l is of opinion that the epithelium dis- 

 appears, and the syncytium is formed by the uterine connective 

 tissue. But it is now generally recognised that the syncytium 

 is trophoblastic. It has been proved by Strahl himself, and by 

 Duval, that many of the villi obtain attachment at parts of the 

 surface where there are no gland openings or crypts, and pene- 

 trate into the substance of the mucosa. Before the disappear- 

 ance of the epithelium, the cells lose their outlines and form 

 a homogeneous mass of protoplasm with fragmented nuclei. 

 This degenerated tissue ought not, as Bonnet 2 emphasised, to 

 be known as syncytium, which represents an active protoplasmic 

 condition (see p. 364). The name which he suggested, symplasma, 

 is very convenient and is used here. It is not only the surface 

 epithelium which forms a symplasma. The glandular epithelium, 

 the connective tissue cells, and extra vasated blood may also give 

 rise to a symplasma which may be designated glandular, con- 

 nective tissue, and hcematogenous respectively. All are formed to 

 a large extent in the placenta of Carnivores, and their resem- 

 blance to the trophoblastic syncytium has led to much confusion. 

 After the destruction of the epithelium, the villi penetrate 

 into the deeper tissues of the mucosa by gradually absorbing 

 the symplasmata, and branch to form secondary and tertiary 

 villi. When the ectoderm reaches the capillary layer, it sends 

 out protoplasmic processes which encircle the dilated vessels. 

 The trophoblast on the sides of the villi becomes syncytial, but 

 retains its cellular character at the tips. Internally the villi 

 contain vascular cores of mesoblast. Hence, is formed the 

 angioplasmode of Duval a continuous layer of foetal vascular 

 villi, clad with syncytium, penetrating everywhere into the 

 capillary layer, and leading to a disappearance of all the maternal 

 tissues except the vessels (Fig. 98). By the epithelial arcades 

 at the tips, the layer of villi rests on the sheet of glandular 

 detritus and the compact layer, which in turn form a symplasma 

 and undergo absorption. Thus the foetal structures reach the 



1 Heinricius, " Ueber die Entwicklung und Struktur der Placenta beim 

 Hunde," Arch.f. mikr. Anat., vol. xxxiii., 1889. 



2 Bonnet, " Ueber Syncytien, Plasmodien und Symplasma," &c., Monats- 

 schr. /. Oeburtsh. u. Oyndk., vol. xviii., 1903. 



