200 Proceedings of the Wernerian Societij. 



being an ingredient not detected in the fossil trunks previously 

 discovered in the same quarry. 



1834, Feb. 1. — Professor Jameson, P. in the chair. — Dr 

 Traill read a memoir of the Reverend George Low, the natu- 

 ralist of Orkney, and laid before the meeting some of his un- 

 published sketches or drawings, and also specimens of his origi- 

 nal notes. 



At this meeting Dr Allen Thomson, at the request of the 

 President, exhibited to the Society, and made remarks on some 

 rare specimens in his possession, illustrating the subject of foetal 

 development in its early stages. 1. Five ova and foetuses of 

 the common cat, which were taken from the mother between the 

 thirteenth and fifteenth day after impregnation. The entire ova 

 are about Jths of an inch in diameter, the foetus is about i^gths. 

 The vascular area on the yolk sac is completely formed ; the 

 intestine still quite open in the abdomen, and the amnios still 

 open over a large part of the back. 2. Two specimens of the 

 foetus of the common fowl, at nearly a corresponding stage of 

 advancement, on the second day of incubation, were shewn to 

 illustrate those" of the cat. 3. The foetus from a swan's egg, in- 

 cubated in an apparatus for between nine and ten days, and cor- 

 responding in its advancement to the chick in ovo at the end of 

 the fourth day. This specimen, from its comparatively larga 

 size, serves very well to shew the branchial slits in the neck, the 

 incomplete state of separation of the ventricles of the heart, and 

 several other peculiarities of structure belonging to this transi- 

 tory period of foetal life. The vesicular o.llantoid membrane 

 attached to the foetus is also very well displayed. 4. A prepa- 

 ration described by Dr Thomson as very rare (there being only 

 two other cases on record), of a double monstrous foetus obtain- 

 ed from a goose's egg, hatched artificially for about five days,, 

 and corresponding in its state of advancement with the chick in 

 ovo towards the end of the second day. This is an instance of 

 a double monster produced from a double germ ; for both foe- 

 tuses, the heads of which lie across one another, are placed in 

 the centre of a single transparent and vascular area; the trans- 

 parent area being of a cruciforn^ shape, as described by Baer, 

 in a similar example above referred to, and the tails of the foe- 

 tuses diverging from one another to each side. The head and 

 spinal column are formed, but the -intestine seems to be as yet 

 quite open, and th^ amniotic folds scarcely begun. There ap- 

 pears to be a single heart common to the two foetuses, situated 

 between them in the angle formed by the neck's crossing. 



At the same meeting was read a notice by Mr Stark, regard- 

 ino- the Mytilus polymorphus of Pallas, a colony of which was 

 lately detected in the Union Canal, not far from Edinburgh. 



