78 



SCIENCE PROGRESS 



reptilian foot already mentioned ; a week later the great toe 

 has begun to approximate to the line of axis of the middle toe ; 

 and in the ninth week this digit has assumed its commanding 

 position and lies nearly parallel with the cartilage of the middle 

 toe. The characteristic human tarsal arch is apparent a week 

 or two later. There is thus no indication of the assumed course 

 of evolution from a foot with the great toe at right angles to 

 the line of the other toes. The transition from the reptilian 

 to the human type proceeds direct without retracing 

 any step. 



A feature which has been prominent in the foot since very 

 early times is the styloid process of the fifth metatarsal bone. 

 This bone is specially associated with the lesser peronei muscles, 



m^> 



Fig. 2. 



Development of Right Foot at s, 6, and 9 weeks. Length of Embryo — } in., i in., and 2 in. 



From Keibel and Mall. 



the third of which is an exclusive human possession and one 

 that appears early in foetal life. The styloid process is 

 important in many marsupials, and was of very large size in 

 the extinct diprotodon. It is mentioned here as an example 

 of the tenacity with which cell-memory holds on to useful 

 structures and often exploits them for new uses. In man the 

 styloid process sometimes asserts its lineage by having a 

 separate centre of ossification for itself. 



We do not know what was the shape of the foot of Ana- 

 ptomorphus or other very early primate. Phenacodus primsevus 

 of the Lower Eocene was near the ancestral line of all the 

 ungulates, and therefore the three middle toes had already 

 become far enough specialised to commit its progeny to the 

 choice of hoofs. From Cope's figure it will be seen that the 

 tarsal bones and the great and little toes are suggestive of 

 the origin of the feet of all the lemurs, monkeys, apes, and man 



