FCETAL NUTRITION: THE PLACENTA 493 



In all the early specimens the ovum was completely enclosed in 

 the uterine mucosa, and the actual process of embedding has not 

 yet been observed. John Hunter considered that the ovum reached 

 the uterus from the Fallopian tube under the mucous membrane, 

 and so had a decidua reflexa, while at a later stage the mucosa 

 developed underneath it ; hence the term decidua serotina. Sharpey 

 supposed that the enclosure was effected by circumvallation, i.e. 

 by a growth round the ovum of two folds of mucosal tissue, which 

 fused and formed the decidua capsularis. But v. Spee 1 discovered a 

 different mode of embedding in the guinea-pig, and later stated that 

 it was the same in man, viz. a destruction of the superficial epithelium, 

 and the 'implantation of the ovum in the cellular substance of the 

 mucous membrane. This view has received considerable support 

 from the researches of v. Heukelom, 2 Peters, 3 Bryce and Teacher, and 

 others. At the same time it must be borne in mind that His, 4 

 in describing an early human ovum in 1897, stated that the 

 implantation cavity was lined with epithelium, 5 and thus represented 

 a part of the uterine lumen shut off by the growth of decidual folds. 



At the time of embedding, segmentation has probably finished 

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

 epiblastic wall disintegrates the epithelium, 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 " G-ewebspilz," filled up at first by a 

 blood-clot which afterwards becomes fibrinous (Peters), and later by 

 decidual tissue (Kollmami 6 ). In Peters' ovum the gap was four- 

 fifths of a millimetre in diameter, and in Bryce and Teacher's a tenth 

 of a millimetre. The size of the ovum when it becomes embedded 

 is probably, according to the last-named authors, a fifth of a 

 millimetre. 



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 proliferation of mesoblast occurs (Fig. 148). In 

 the youngest ovum its cells filled the space between the wall of the 



1 V. Spee, " Neue Beobachtungen iiber sehr friihe Entwicklungsstufen des 

 menschlichen Eies," Arch. f. Anat. u. P/iys., Anat. Abth:, 1896. 



2 V. Heukelom, "Ueber die menschliche Placentation," Arch. f. Anat. u. Phys., 

 Anat. Abth., 1898. 



3 Peters, Ueber die Einbettunq des menschlichen Eies, Leipzig u. Wien, 1899. 



4 His, " Die Umschliessung des menschlichen Frucht wahrend der friihesten 

 Zeit der Schwangerschaft," Arch.f. Anat. u. P/n/s., Anat. Abth, 1897. 



5 Cf. Johnstone, " Contribution to the Study of the Early Human Ovum," 

 Jour, of Obstetrics and Gyncecology, (May) 1914. 



Kollmann, "Die menschlichen Eier von 6 Millimeter Grosse," Arc/i. f. 

 Anat. u. P/it/s., Anat. Abth., 1879. 



