584 PHILOSOPHICAL TRANSACTIONS. [aNNO I7OI. 



H below the line ee was alike in transparency with that part of the allantois d 

 above it. Whereas had the amnios been still joined to the allantois, as the ob- 

 jection supposes, the allantois below this line must have appeared thicker than 

 that part above it, since the amnios alone is much thicker than the allantois. 

 It is easy indeed to conceive the amnios running an entire bladder, or membrane, 

 under the allantois, and perhaps it may be so. 



Others have taken this allantois to be an amnios of one of the twins belong- 

 ing to these secundines. This objection, though it may seem plausible, yet is 

 of no force. For 1st, this allantois is much finer to the touch, and also much 

 more transparent than the other amnios ; which still remains stiff", while the 

 much thinner allantois sinks on the least blast of air, notwithstanding the styles 

 II which assist it. 2dly, This allantois had two visible urachi, and it is of an 

 oviform figure, somewhat resembling the common figure of a man's bladder. 

 Also this allantois no where touches the placenta, unless at the neck f. But on 

 the contrary, the amnios is of the same irregular figure, as the position, 

 motion, &c. of the foetus require. Likewise, it covers the whole internal 

 surface of the placenta. 3dly, They who make this objection must suppose 

 some hole in this bladder, and in the amnios, through which one umbilical cord 

 may pass from the placenta to the foetus. But such a foramen would be preter- 

 natural, because the navel-string runs only from the placenta to the foetus, under 

 a coat taken from the amnios, and lies with the foetus in the cavity of the am- 

 nios, which is no where perforated. 4thly, The hole at the fundus g was 

 scarcely wide enough to receive the end of a man's finger, whereas the twins 

 did not want six weeks of their full time. Since therefore a foetus of near eight 

 months could not possibly pass this orifice, this bladder could not be an amnios. 



Nothing in these secundines is preternatural, only some things were not ob- 

 served before. Hitherto anatomists have not allowed twins to lie in a common 

 amnios, but supposed each foetus to have a distinct one. Tlie reason of this 

 opinion might be, that some denying any urinary membrane, called every mem- 

 brane they found, except the chorion, an amnios ; and ihese finding two mem- 

 branes in the secundines of twins, supposed them to be two amnii. That others 

 granting an allantois, but not distinctly discovering it, but only two membranes, 

 also imagined them to be two amnii. Both of these taking that for an amnios, 

 which might really be an allantois. But since one chorion, and one placenta, 

 these being ever of the same number, generally serve twins, nay, sometimes 

 three foetuses, why should it seem strange, that one amnios, at least sometimes, 

 and one allantois, should serve the like number } 



I am not ignorant that Mauriceau, and Diemerbroeck, think there is an ab 

 solute necessity for every foetus to lie in a distinct amnios, and that otherwise 



