FCETAL NUTRITION: THE PLACENTA 467 



finished and the ovum is in the condition of the early blastocyst. 

 Its epiblastic wall disintegrates the epithehum, the subjacent 

 cells, and a few capillaries at the point of contact. Hence the 

 blastocyst comes to lie in the connective tissue of the mucosa, 

 which completely surrounds it, except at the point of entrance of 

 the ovum. Here there is a gap in the tissue, the " Gewebspilz," 

 filled up at first by a blood-clot which afterwards becomes 

 fibrinous (Peters), and later by decidual tissue (KoUmann ^). 

 In Peters' ovum the gap was four-fifths of a millimetre in 

 diameter, and in Bryce and Teacher's a tenth of a milhmetre. 

 The size of the ovum when it becomes embedded is probably, 

 according to the last-named authors, a fifth of a milhmetre. 



When the hypoblast of the early blastocyst is differentiated, 

 it does not apparently line the wall of the blastocyst, but forms 

 a small vesicle. Very early, even before the appearance of the 

 primitive streak, a marked proUferation of mesoblast occurs 

 (Fig. 121). In the youngest ovum its cells filled the space 

 between the wall of the blastocyst and the small amniotic and 

 hypoblastic vesicles. In the ovum described by Leopold,^ it was 

 aheady spht by the " Haftstiel " into two parts, which enclosed 

 the coelom and were continuous with each other (Fig. 122). The 

 outer waU of the blastocyst, the foetal ectoderm or trophoblast 

 which anchors the ovum in the mucosa, is thickened all round 

 its circumference, and even in the earhest specimen contained 

 vacuoles into some of which maternal blood had penetrated. In 

 this thick spongy layer Bryce and Teacher found no cell-outlines 

 anywhere. Hence the transformation to syncytium is not due, as 

 Peters supposed, to the contact with maternal blood. Under the 

 syncytium is the cellular layer, corresponding to the cytoblast of 

 Beneden. Its cells are in a state of active division, and they 

 appear later to lose their outhnes and merge into the. syncytium. 

 The growth of the latter from the mother-zone of cytoblast occurs, 

 not as a sohd mass, but in strands forming primitive syncytial 

 viUi (Fig. 123). Into the syncytium project outgrowths of the 

 cytoblast, forming the cellular viUi of Peters and Leopold. In the 



' KoUmann, "Die menschliohen Eier von 6 Millimeter Grosse," Arch, 

 f. Anat. u. Phys., anat. Ahfh., 1879. 



' Leopold, " Demonstration eines sehr jungen menschliohen "Eiies," Arbeiten 

 aus d. Kbnigl. Frauenklinik in Dresden, Leipzig, 1906. 



