Bo j anus opened only the external membrane of the chorion, 

 and hence he was able to see the actual location of the 

 embryonic membranes and organs. His predecessors had not 

 taken the necessary precaution's and had come to a number of 

 incorrect conclusions, in particular to the determination that 

 the umbilical sac is located inside the allantois. Bojanus 

 accurately determined the shape and topography of the umbilical 

 vesicle (tunica erythroides, vesicula umbilicalis) . The latter 

 has the shape of a folded, reddish Cdue to a great number of 

 vessels), thin stem which extends from the umbilical cord 

 and is stretched between the main vessel stems which connect 

 the umbilical cord with the belt of placenta. The umbilical 

 vesicle spreads along the fetus to the end of the fetal sac, 

 where it is joined with chorion Cand not with the amnion, as 

 was thought by Needham and Oken) . Daubenton likewise did not 

 understand the nature of this formation and wrongly considered 

 it to be the allantois, which is not surprising because he did 

 not find any connection with the urinary bladder. Besides the 

 umbilical vesicle, at this stage the allantoic canal appears, 

 though its detection is not easy. 



In order to reach the allantois and the amnion, it is 

 necessary to open the internal membrane of the chorion with 

 its thin blood vessels. With careful preparation, the removal 

 of this membrane does not lead to the pouring off of the 

 fetal fluid. Such a careful opening of the internal membrane of 

 the chorion is difficult to accomplish, however, because the 

 deep membranes are firmly attached with it and are very thin. 



Successful opening allowed Bojanus to correct the 

 prevailing wrong belief in that time, that the amnion and 

 the embryo are located in the allantoic cavity and swim in 

 the fluid filling it. The displacement of the fetus in the 

 amnion by pressing on it from the outside and by blowing the 

 allantois with air after releasing its fluid through a small 

 cut, allowed Bojanus to determine that the amnion is not 

 surrounded on all sides with the allantois. Instead, the latter 

 forms around the fetus which lies in the amnion. 



The results of Bojanus' investigations are summarized 

 in the following manner. The fetus of the dog has four 

 membranes: the amnion, allantois, umbilical sac and chorion. 

 The amnion lies not in the cavity of the allantois, but near 

 the sac formed by it; the amnion is surrounded by the allantois 



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