VOL. XXII.] PHILOSOPHICAL TRANSACTIONS. 585 



twins in the same membrane would grow together, and make a monster. 

 Aquapendens further says, that all ova gemellifica produce some other sort of 

 monster. Yet it is most certain that ova gemellifica exclude two perfect 

 chickens, though not both alive. Dr. Harvey indeed thinks it possible, that 

 such an ovum may produce a monstrous chicken, if its vitelli be contained in 

 the same membrane, &c. yet he does not positively say it must be so. For my 

 part, I cannot see any more reason why twins in one amnios should grow to- 

 gether, than that the hands or heels of the same foetus should grow to its own 

 body. How can the humours that lubricate a single foetus, and help it to move, 

 join two together ? since the humours are the same, and the parts of the same 

 foetus, as tender as those of twins are, lie as close to each other as twins 

 do. It is very observable, that among all the monsters we read of, there are 

 very few, which seem to be made of two entire bodies joined together ; and that 

 most of these, on dissection, were found to have only one heart, one liver, &c. 

 whence it is evident, that these monsters, and no doubt all others, were origi- 

 nally monsters in the ova, before impregnation, and not so from the want of 

 an amnios. Yet Diemerbroeck does not a little boast of having first, as he 

 thinks, found the reason, why twins must lie in distinct amnii. But since the 

 matter in question (sometimes at least, as in these secundines, where there was 

 only 1 amnios and 2 regular foetuses) is not true, his argument for a ne- 

 cessity of two amnii for two foetuses will never prove valid, even where twins, 

 and two amnii are found. Indeed any part may be made to grow to any part, 

 as we see in the cure of hair lips, &c. but then the fibres must be first broken, 

 before there can be any union. Now I cannot conceive what should naturally 

 break the fibres of the twins in the uterus. But although it is evident from 

 what has been said, that twins may lie distinct in the same amnios, yet there 

 must be as many urachi as foetuses. In these secundines I saw two running 

 over the placenta to the neck of the allantois. The urachus passes under the 

 amnios, as the other umbilical vessels do, and runs from that part where the 

 umbilical cord is joined to the placenta, straight to the cervix p. s describes the 

 course of that urachus, marked r at f in the 3d fig. The other urachus lay 

 about a quarter of an inch laterally beyond that marked r in the same figure. 

 I mean by two urachi, two long roundish bodies, of a depressed figure ; they 

 seemed as large as a common knitting-needle, and were of a darker substance 

 than the placenta on which they lay. They appeared in every respect like that 

 part of the navel-string which is allowed by all anatomists to be the urachus, 

 and in like manner shrunk in two or three days, from a mucous substance to a 

 mere membrane. 



These two are the only entire urinary membranes that I have prepared. Yet 



VOL. IV. 4 F 



