EARLIEST HUMAN OVA. 711 



unimpregnated women who died at the time cf the invasion of tiie 

 menstrual flow, Mr. H. Letheby detected an ovum covered by the 

 granuhu- cells in the first part of the Fallopian tube, on the same side 

 on which a ruptured follicle was found in the ovary (Trans. Roy. Soc. 

 Lond. 1852, p. 7). But the fecundated human ovum has not yet been 

 traced in the course of its descent in the Fallopian tube, nor has it, 

 indeed, been seen in a satisfactory manner previous to the time when it 

 is already imbedded in the uterine decidua. The history of the most 

 authentic cases of the earliest ova observed in this state leads to the 

 conclusion that the fecundated human ovum does not reach the cavity 

 of the uterus before the seventh or eighth day after its escape from the 

 ovary ; and that already by the twelfth or thirteenth day, if not even 

 sooner, it has acquired a chorion covered with villi, and has become 

 imbedded in the decidua. 



The human ovum, like that of mammals generally, probably under- 

 goes very little enlargement during its descent through the tube and 

 the occurrence of segmentation, and its diameter on arriving in the 

 cavity of the uterus does not probably surpass one hundredth, or at 

 most one eightieth of an inch'. A very rapid expansion, however, of 

 the whole ovum no doubt occurs immediately after its entrance into 

 the uterus, in the same manner as observed in a number of mammals, 

 so that within two or three days, its size may have increased to ten 

 or twelve times its original diameter. Having at first only the zona 

 pellucida for its external envelope (or primitive chorion), and being 

 nearly smooth on the surface, it soon acquires, by a process previously 

 described, its new chorion, on which are formed the permanent villi ; 

 and by the time when it has become incapsulated, according to 

 Reichert's observations in the recently published very careful descrip- 

 tion of an ovum which was probably of the thirteenth or fourteenth 

 day after impregnation, it has a diameter of nearly one-fourth of an 

 inch, and is beset with villi over a considerable part, but not the whole 

 of its surface (see fig. 517). 



Several other examples of incapsulated ova of nearly the same period 

 have been observed, as by Von Baer, Velpeau, Wharton Jones, Coste, 

 and Allen Thomson ; but in all these, with the exception of one 

 observed by Velpeau, and supposed to be of the tenth day, or earlier, 

 the whole surface of the ovum was already uniformly covered with short 

 thick set chorionic villi. 



The ova observed by Velpeau and Wharton Jones were like that described by 

 Reichert of a period previous to the formation of an embiyo, and may be stated 

 as probably of from ten to foui-teen days old. (Velpeau, " Ovologie Hiimaine," 

 Paris, 1833 ; WTiarton Jones ia '• Trans. Roy. Soc. of Lond.," 1837, p. 339 ; Reichert, 

 " Beschr. einer Friihzeit. Menschl. Frucht, &c.," Berlin, 1873.) Those of Von Baer, 

 Pockels, Coste, and Allen Thomson are of the period immediately following, or 

 from fourteen to eighteen days after impregnation. (Von Baer, " Entwickelungs- 

 geschichte," p. 270 ; Pockels in "Oken"s Isis," 182.5 ; Coste, " Hist. Gen. et Partic. 

 du Developpement," 1817 ; Allen Tliomson, '• Contrib. to the Hist., &c., of the 

 Human Ovum before the Third Week after Conception," in " Edin. Med. and 

 Sui'g. Joum." No. 140.) 



Formation of Decidua. — Before the arrival of the ovum in the 

 uterus, the lining membrane of that cavity undoubtedly undergoes a 

 preparatory change, by which the formation of the decidua is com- 

 menced, and this change, in its first stages, or up to the time of the 



