OF ARTS AND SCIENCES. 81 



Professor Jeffries Wyman, exhibiting a stereoscopic view of 

 the skeleton of a double human foetus, discussed the question 

 of the mode of origin of such monstrosities, and insisted that 

 they never arose from actual coalescence of two individuals, 

 but from the more or less extensive longitudinal division, or 

 rather bifurcation, of the primitive stripe of the ovum, with 

 which the development of the embryo begins. He was thus 

 led to consider the question of individuality, and to maintain 

 the ground that, since the two bodies or parts of bodies were 

 not formed by the coalescence of two originally distinct primi- 

 tive stripes, therefore they were to be regarded as one individ- 

 ual, even in a case so extreme as that of the Siamese twins. 



This view was criticised by Professors Parsons, Bowen, and 

 Gray, the latter assenting to this view of the origination of 

 such double individuals, as agreeing with the chorisis or 

 similar doubling of organs in the vegetable kingdom ; but 

 insisting that to call the Siamese twins one individual was a 

 practical reductio ad absurdum of that idea of individuality, 

 and that individuality should be considered as of complete 

 or incomplete realization ; e. g. that a bicephalous monster 

 was the result of an incomplete development, the Siamese 

 twins, of an essentially effectual development of two individ- 

 uals out of the foundation of one, or in the normal place 

 of one. 



Dr. C. Pickering submitted a statement relative to the geo- 

 graphical distribution of species, viz. : — 



That his experience as a naturalist had led him to the conclusion, 

 that the main limiting cause in the diffusion of species is to be found 

 in the envelope of the ovum ; in other words, the shell of the ovum 

 governs the diffusion of species. 



When the shell of the ovum breaks before exclusion, as in animals 

 called viviparous, the species cannot be diffused by means of ova. 



Other organic beings capable of locomotion are diffused both by 

 ova and the wandering progeny ; but plants are diffused exclusively 

 by ova. 



Change the order of Nature ; let the ova of insects be all borne 

 VOL. V. 11 



