THE F(ETAL MEMBRANES OF MAN. 241 



CHAPTER XIII. 



THE FCETAL ME MB RAXES OF MAN. 



THE investigation of the first stages in the development of man, 

 which are accomplished during the first four weeks of pregnancy, is 

 coupled with extraordinary difficulties. Only very exceptionally does 

 the embryologist come into possession of young human ova, whether 

 found in the uterus at the time of dissection, or coming into the 

 hands of a physician as the result of miscarriage. In the latter case 

 the ova have often been dead for a long time in the uterus, and 

 consequently are in process of decomposition. Finally, a good 

 preservation and an accurate investigation of such small and 

 delicate objects demand no slight degree of -skill. 



This accounts for the fact that we do not possess in the case of 

 Man a single observation upon the process of fertilisation or that of 

 cleavage, upon the formation of the germ-layers, or upon the first 

 establishment of the form of the body, i the foetal membranes, and a 

 large number of other organs. Concerning this whole period we 

 are dependent upon the conclusions which are furnished by the 

 development of other Mammals. Thus we assume that fertilisation 

 normally takes place in the enlarged beginning of the oviduct 

 (Fallopian tube) ; that the seminal elements, which remain alive in 

 the female sexual organs perhaps for days or weeks, here await the 

 ovum as it emerges from the ovary ; that the ovum already segmented 

 enters into the cavity of the uterus, attaches itself in the mucous 

 membrane, and during the first weeks of pregnancy gives rise to the 

 germ-layers, the outer form of the body, and the foetal membranes, 

 according to the well-known rules for other Mammals. 



A little, although very scanty, information has been acquired, 

 but this concerns only the second and subsequent week. A small 

 number of ova have been described in the literature, which for the 

 most part come from miscarriages, and the age of which has been 

 estimated at from twelve to fifteen days. The blastodermic vesicles 

 measured 5 to 6 mm. in diameter. Here belong two ova described 

 by ALLEN THOMSON, and those by SCHRODER v. D. KOLK, HENNIG, 

 REICHERT, BREUSS, BEIGEL UND LOWE, as well as the cases published 

 by AHLFELD, KOLLMANN, FOL, and GRAF SPEE. 



Upon critical comparison of the discoveries, there are two facts 

 which we can regard as established. 



First. At the end of the second week the blastodermic vesicle 



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