REPORT ON THE MARSUPIALIA. 139 



different periods of development. In the first the metatarsal bones, with the exception 

 of the first two, are in close apposition, and in consequence all the interosseous muscles, 

 excepting the first dorsal, are plantar in position. The second diagram is from a foot 

 somewhat more advanced. It shows that as development progresses the metatarsal bones 

 separate from each other, and that simultaneously with this the dorsal interossei begin to 

 shoot up between them like wedges. The third illustration gives a view of the relative 

 position of the muscles and metatarsal bones as they are to be seen in the adult. The bones 

 are widely apart from each other, and the muscles have reached the dorsum of the foot. 



Among the lower mammals there are many animals which in their adult condition 

 correspond exactly with the first stage of the human embryo in the relation of their 

 intrinsic pedal muscles to the metatarsus. Perhaps the Dog and the Dasyure are the 

 best examples that I could quote. In these the metatarsals are closely compressed 

 together, and the muscles are entirely plantar in position (PL VIII. fig. 4). 



The majority of animals never reach beyond the second stage of the human embryo 

 in this respect. Let us take the foot of the Leopard as an example. In this animal, 

 the metatarsal bones, whilst they are closely applied to each other towards the tarsus, 

 open out slightly from each other towards their phalangeal extremities ; and in the 

 intervals between them the thin sharp edges of the dorsal interossei may be seen reaching 

 half-way up the interosseous spaces (PI. VIII. fig. 7). 



Comparatively few animals correspond with the third or adult condition of the 

 human foot ; still certain of the Apes approach very closely to Man in this respect. 1 



It is curious to find in an animal so low as the Duck-bill Platypus a closer approach 

 to Man than in the vast majority of mammals. The dorsal interossei reach the dorsum of 

 the foot, and, moreover, the second and third are two-headed. 



But there is also a relation between the human embryo and many of the adult 

 animals in the mode of origin of the dorsal interossei. Euge points out that in the early 

 embryo these muscles are one-headed, and that it is only in a later stage that they 

 acquire their bipenniform character and their origin from the metatarsal shafts. How 

 similar is this to what we have seen to be the permanent condition in the great majority 

 of mammals. 



1 Mr. Chanipneys, when he wrote his well known paper upon the Chimpanzee and Anubis (Jour. Anat. and Phys., 

 vol. vi. p. 207), had a very clear conception of the different conditions which the intrinsic pedal muscles presented to 

 the metatarsus. The following is a footnote appended to his paper : — " Duvernoy remarks that in the Gorilla, as I 

 found in Chimpanzee, the dorsal interossei are not so dorsally placed as in Man. Moreover, this fact was plainly sit 

 forth in the hand of my Chimpanzee, in which parts of the dorsal interossei were so divaricated palmarly as to be 

 positively palmarly, and not at all dorsally, placed. In the lower Monkeys, as Cebus, Innuus nenustrinus, and in 

 Anubis, there are really no true dorsal interossei, but two layers of plantar, the more dorsally lying of which we hmy 

 take, if we please, to represent the dorsal interossei. The more plantarly placed resemble the interossei of the 

 Carnivora, as Church remarks. We, therefore, have an ascending series from that case where the dorsal interossei are 

 plantarly placed (represented by the Cebus and Anubis, the more plantarly placed resembling those of the Carnii 

 to the Anthropoid Apes, where these are more dorsal, and thus to Man, where they are more dorsal still. This dorsal 

 migration of the interossei is very interesting." 



