46 



HUMAN ANATOMY. 



Diagrams representing relations of the uterine mucous mem- 

 brane to the embryonic vesicle, or ovum, during tlie embedding 

 of the latter, j-, v, c, decidua serotina, vera, arid reflexa, re- 

 spectively ; o, ovum. 



cavity and becoming arrested at some favorable point, usually in the vicinity of 

 the oviduct, brings about a degeneration of the uterine epithelium over the area of 

 contact. The disappearance of the epithelial lining is followed by sinking and em- 

 bedding of the embryonic vesicle within the softened mucous membrane, the process 

 being accompanied by erosion of some of the uterine capillaries and consequent 

 hemorrhage into the opening representing the path of the ovum. The extravasated 

 blood escapes at the point of entrance on the uterine surface and, later, forms a 



mushroom-shaped plug marking the 

 Fig. 52. position of the embedded ovum. 



The latter thus comes into closer 

 relations with the maternal tissues at 

 an earlier period than was formerly 

 recognized. 



The Trophoblast.— The ear- 

 liest human embryonic vesicle that 

 has been accurately studied, — that 

 of Bryce and Teacher, — measuring 

 only I millimetre in its greatest di- 

 ameter, was already enclosed exter- 

 nally by a conspicuous ectoblastic 

 envelope, consisting of an outer and 

 an inner cell-layer. This thick 

 ectoblastic layer is evidently the proliferated trophoblast (.page 31), a membrane so 

 designated to indicate the important nutritive functions which it early assumes. 



Very early the trophoblast becomes honeycombed by the extension of the 

 maternal vascular channels into the ectoblastic tissue (Fig. 53), which consequently 

 is broken up into irregular epithelial trabeculae separating the maternal blood-spaces. 

 The inner surface of the trophoblastic capsule presents numerous irregular depres- 

 sions into which corresponding processes of the adjacent young mesoblast project ; 

 this arrans^ement foreshadows the formation of the chorionic villi which soon become 

 so conspicuous in the human embryonic vesicle. Coincidently with the invasion of 

 the trophoblast by the vascular lacuna 



externally and the penetration of the Fig. 53. 



mesoblastic tissue internally, the pe- 

 ripheral portions of the ectoblastic 

 capsule undergo proliferation and 

 extend more deeply into the sur- 

 rounding maternal tissues. In con- 

 sequence of the rapid growth of the 

 embryonic vesicle, that part of the 

 hypertrophied uterine mucosa which 

 overlies the embedded embryonic 

 vesicle soon becomes elevated and 

 projects into the uterine cavity, thus 

 giving rise to the structure described 

 as the decidua reflexa, or, preferably, 

 the decidua capsular is. 



The Decidua Vera. — The 

 changes which affect the uterine mu- 

 cous membrane, the decidua vera, 



result in great thickening, so that the mucosa often measures nearly a centimetre ; 

 this thickening, however, is most marked in the immediate vicinity of the embedded 

 ovum, throughout the greater part of the uterus the decidua attaining a much less 

 conspicuous hypertrophy. Towards the cervix the mucosa is least affected, and at 

 the internal orifice of the cervical canal presents its normal appearance. Examina- 

 tion of the decidua shows that the normal constituents of the uterine mucosa undergo 

 hypertrophy which results in enlargement of the uterine glands (Fig. 54), as well 

 as in increase of the intervening connective-tissue stroma. The enlargement of the 

 glands is not uniform, but is limited to the middle and terminal or deeper parts of 



Mesoblast 



Trophoblast 



Intervillous 

 blood-space 



Svncvtium 



Maternal 

 blood-space 



^—Uterine 

 -" muscle 



Diagram showing early stage of attachment between foeta| 

 and maternal tissues; invasion of trophoblast by maternal 

 blood-vessels. (Peters.) 



