THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 217 



The Carpal Bones. — Occasionally an os centrale is present in addition to 

 the usual carpal bones. It is situated on the dorsal side of the wrist between the 

 navicular, capitate and small multangulum. In the embryo an additional piece 

 of cartilage is of constant occurrence in this location, but usually disappears 

 during later development; in cases where it persists, ossification takes place 

 to form the os centrale. In some of the apes the os centrale is of constant 

 occurrence in the adult. 



The Femur. — The gluteal tuberosity (ridge) sometimes projects like a 

 comb, forming the so-called third trochanter, a structure homologous with the 

 third trochanter in the horse and some other mammals. 



The Tarsal Bones. — Cases have been recorded in which the total number 

 of tarsal bones was reduced, owing to congenital synosteosis (fusion) of the 

 calcaneus (os calcis) and scaphoid (navicular), of the talus (astragalus) and 

 calcaneus, or of the talus and scaphoid. Occasionally an additional bone — the 

 trigonum — is present at the back of the talus. In the embryo, the talus ossifies 

 from two centers which normally fuse at an early stage into a single center. 

 The trigonum probably represents a bone produced by one of the centers which 

 has remained separate. 



Polydactyly. — This anomaly consists of an increase in the number of 

 fingers or toes, or both. Any degree of variation may exist from a supernum- 

 erary finger or toe to a double complement of fingers or toes. The causes under- 

 lying the origin of such anomalies are not clear. Some assign the supernumer- 

 ary digits to the category of pathological growths or neoplasms, linking them 

 with partial duplicate formations. Others explain the extra digits on the ground 

 of atavism or reversion to an ancestral type. The latter explanation assumes 

 an ancestral type with more than five digits. But neither zoology nor paleon- 

 tology has found any vertebrate form, above the Fishes, which normally pos- 

 sesses more than five digits on each extremity. Consequently one must refer to 

 the Fishes for some ancestral type to explain the existence of more than five 

 digits. Going back so far in phylogenetic history, no certainty whatever can be 

 attached to the origin of supernumerary digits, for it is not even known from 

 what fins the extremities of the higher forms are derived. Still another view 

 regarding the origin of supernumerary digits is that they are due to certain ex- 

 ternal influences among which the most important is the mechanical impression 

 of amniotic folds or bands. This, however, could not be the sole cause of 

 polydactylism, since such malformations are common in amphibian embryos 

 where no amnion is present. 



PRACTICAL SUGGESTIONS. 



Embryonic Connective Tissue. — The most easily obtained material for the study of the 

 development of embryonic connective tissue is the chick embryo. From about the beginning 

 of the second day to the end of the third day of incubation, the differentiation of the meso- 



