G06 THE HUMAN EMBRYO. 



preparation on the part of the uterus for an ovum which m 

 reaches it; the decidua, after waiting a certain time, becoriiino- 

 broken up and discharged. If, however, impregnation is effected, 

 and a fertilised ovum reaches the uterus, a new stimulus is set 

 up, and the developmental processes, instead of stopping, go on 

 to further stages, and so give rise to the decidua of pregnancy. 



Prior to the arrival of the ovum in the uterus, the decidua 

 forms a complete lining to the uterus^ It does not cover the 

 orifices of the Fallopian tubes (Fig^ z54), which remain open 

 throughout the greater part or the whole of pregnancy ; neither 

 does it extend into the cervix uteri, but stops abruptly at the 

 os internum. With these exceptions, the decidua forms a layer, 

 of approximately uniform thickness and structure, covering all 

 parts of the uterine wall. 



It seems to be, to a great extent, a matter of chance with what 

 part of the uterus the ovum will come in contact, on entering its 

 cavity ; and it is therefore important that all parts Of the surface 

 should be equally prepared to receive it. In the great majority 

 of cases, the attachment of the ovum is in the neighbourhood of 

 the fundus, usually rather to one side of the median line, and 

 more frequently on the dorsal than the ventral surface. It 

 may, however, be situated in almost any part of the uterus ; and 

 its position may become a point of much practical importance. 

 Ercolani has suggested that the ovum, on entering the uterus, 

 is prevented from at once sinking to the cervix, by the fluid 

 secreted by the utricular glands of the uterus, and that it floats 

 on the surface of this fluid, until it comes in contact with, and 

 adheres to, the wall of the uterus ; the actual place of contact 

 would in this case vary considerably, according to the amount of 

 fluid in the uterus at the time. 



The youngest ovum yet found in situ within the human 

 uterus, that described by Reichert, was not simply attached to 

 the decidua, but completely embedded in this (cf. Fig. 175); a 

 relation which is retained throughout the whole period of gesta- 

 tion (Fig. 254). 



There has been some discussion as to the mode in which 

 this encapsuling of the ovum is brought about ; there are no 

 direct observations on the point in the case of human embryos, 

 but the fact that the openings of the uterine glands occur on 

 both surfaces of the encapsuling layer of the decidua, together 



