Embryonic Development in Man. 391 



a secretory function, it is possible that this apparatus of the syncy- 

 tium has something to do with the secretion of the hypothetical 

 enzyme of the trophoblast mentioned above. The nuclei of the 

 syncytium are generally oval, elongated and rather densely pro- 

 vided with chromatin. I have, like Bonnet, not been able to find 

 any cuticle or basement membrane between the syncytium and thp 

 cells of the trophoblast. In speaking about the outermost processes 

 of the trophoblast, we described above how the trophoblast cells have 

 invaded the narrow strip of outer polar tissue designated as decidua 

 capsularis. Such processes of proliferating trophoblast elements, in- 

 cluding both cells and syncytial masses, are found extending into the 

 maternal tissues around the whole circumference of the ovum. This 

 zone directly surrounding the ovum, forming the soil into which 

 such invading processes extend, has been called the "Umlagerungs- 

 zone," by Peters, a term which perhaps may be best translated by 

 the ^'Border Zone." 



The Border Zone. 

 The tissue which forms the bed of the ovum (Eilager) surround- 

 ing it more or less from all sides may be divided into three parts, 

 the decidua basalis, the decidua capsularis, and the equatorial zone 

 or decidua vera. The border zone at the base and around the equa- 

 torial planes of the ovum is characterized by the presence of large 

 blood sinuses, originally formed from the capillaries and small 

 veins of the uterine mucosa. Some of these blood lacunae have 

 retained the outlines of vessels; others have become irregular spaces 

 which have no resemblance to ordinary vessels. The largest of these 

 blood sinuses in our specimen are found in the decidua basalis, 

 near the inner pole of the ovum. But very large thin-walled blood 

 spaces surround the ovum on all sides. They proceed from the 

 basal decidua into the equatorial border zone, bend around the upper 

 or outer hemisphere of the ovum, and very nearly reach the thin 

 polar cap of tissue, the decidua capsularis. No real blood spaces 

 are, however, found in this thin cap of tissue, but only free blood 

 corpuscles mixed with cells either of maternal origin (decidual 

 cells) or derived from the ectoblast shell. Very much enlarged capil- 



