292 Studies of the Development of the Human Skeleton 



Cartilagenous ^cno(?.— This Schomburg distinguishes from the pre- 

 ceding by the fact that cartilage cells at the centers of the areas of 

 chondrofication show definite cell boundaries and become larger than the 

 surrounding prochondral cells. These changes take place in the various 

 skeletal anlages in the order in which the anlages were originally formed. 

 With the active production of cartilage cells the broad surrounding zone 

 of mesenchyme gives way to a narrower, denser perichondrium. At the 

 same time the form of the skeleton becomes more definite, so that, as 

 Schomburg says, the cartilages of the foot of an embryo at the middle of 

 the third month give a good picture of the adult bones of the foot. The 

 articular surfaces acquire more or less their definite form. I quite agree 

 with Schomburg, in opposition to Henke and Eeyher, that the joints of 

 the foot, like the other joints of the body, are laid down at the start in 

 their definite form and are not moulded into shape by use. 



The skeleton of the foot at the time when the cartilage cells at the 

 centers in most of the bones are beginning to be distinctly outlined, has 

 the form shown in Figs. 7, 8 and 59. The tibia is much larger than the 

 fibula and extends further distally. The astragalus has somewhat the 

 form of a rhomboid plate which runs dorsally from the fibular side 

 toward the tibial side on the plantar surface. The calcaneus is rather 

 small and is in direct line with the long axis of the fibula but in a plane 

 lying further plantarwards. The navicular is in a direct line with the 

 astragalus. Its tibial edge lies near the lower end of the tibia. The 

 three cuneiform bones are proportionately broader and thicker than in 

 the adult skeleton. The cuboid is in direct line with the calcaneus. 

 The metatarsals lie less spread apart than at an earlier stage, Figs. 5 

 and 6. The first phalanx has developed in all of the toes, and in the 

 second toe, the second phalanx as well. At the region of the phalangeal 

 joints there is a swelling of the blastemal tissue. 



If now these figures be compared with Figs. 9 and 10, which show the 

 foot of an embryo of 20 mm., the most noticeable change will be seen in 

 the astragalus. This has become considerably thicker. It extends fur- 

 ther than the calcaneus. Between the til)ia and the navicular it has so 

 increased in size that the foot is bent toward the fibular side. A much 

 greater interval than in Embryo XVII, Figs. 7, 8 and 59, exists between 

 the two bones. 



The calcaneus has extended considerably in length both in a proximal 

 and in a distal direction. The cuneiform bones are becoming crowded 

 together. The cuboid is larger than in XVII. The phalanges are at a 

 similar stage of development. The joints between the metatarsals and 

 phalanges are surrounded by a mass of dense tissue, while the tissue 

 of the joints themselves is of a light texture and resembles prochondrium. 



