518 A. A. W. HUBREOHT. 



vessels that are already present in the maternal proliferation, 

 which, as we have seen on p. 516, are also very markedly spread 

 out horizontally below the layer of epithelium, which has 

 come to disappear now that the trophoblast has taken its 

 place. Out of this immediate contiguity an actual communi- 

 cation of the maternal blood-spaces with those in the plasmo- 

 diblast is very soon developed, as can be clearly seen in fig. 80. 

 The plasmodiblast does not henceforth develop independently 

 of the cytoblast ; on the contrary, it is continually being added 

 to by cells or cell sheets from the darker stained cytoblastic 

 layer getting more detached and travelling inwards. This is 

 clearly indicated in fig. 93, which has reference to the uterus 

 No. 85, i. e. one stage later than (fig. 12) uterus No. 3 (fig. 

 11), from which fig. 80 is taken. In fig. 93 the detachment of 

 cytoblastic elements from the more deeply stained layer that 

 arrange themselves into parallel superposed layers of plasmodi- 

 blast, between which open spaces develop, in which blood 

 gradually penetrates, is very distinctly visible, as it is also in 

 numerous preparations of this and the earlier stages that have 

 not here been figured. 



As further growth goes on, these horizontal blood tracts 

 assume a more vertical course, new horizontal ones developing 

 below them (cf. fig. 89). 



Also in fig. 87 the penetration of blood in trophoblastic 

 spaces is indicated, — this time in a section that cuts the pla- 

 centa circularly (cf. fig. lib). 



Thus, through the action of the massive trophoblast, not 

 only a firm attachment of the blastocyst in the placentary 

 region is brought about, but between the trophoblastic knobs 

 which have caused that firm attachment a sanguiniferous plas- 

 modiblast is very early established. A.llantoidean villi pene- 

 trating into the trophoblastic knobs that become hollowed out 

 as they lengthen centripetally are thus bathed by maternal 

 blood circulating in embryonic spaces. 



There is thus neither necessity nor even possibility for any 

 further penetration of proliferating maternal tissue between 

 the villi. That space is filled by the allantoidean trophoblast. 



