282 Studies of the Development of the II u man Skeleton 



While the hnman embryo is growing from 15 to 20 mm. in length, 

 there occnrs a rapid development of the pelvic cartilages. About the 

 head of the femur each gives rise to a plate-like process. The fusion of 

 these processes produces a shallow acetabulum, Figs. 9 and 10, Plate IV. 

 Those from the ilium and ischium are larger than that of the pubis and 

 fuse with one another before the pubic cartilage fuses with them. The 

 proportional areas of the acetabulum to which each pelvic cartilage con- 

 tributes seem to be essentially the same as those later furnished by the 

 corresponding pelvic bones, % -|- ischium, % — ilium, % pubis. AVhile 

 growing about the hip-joint so as to complete the acetabulum each of the 

 pelvic cartilages has a centrifugal growth within the blastemal pelvis. 

 It is convenient to consider each cartilage in turn. 



The iliac cartilage of the stage represented in Embryo CXLIV, Figs. 

 5 and 6, represents essentially that portion of the ilium which borders 

 the entrance to the true pelvis and which has hence been called the pelvic 

 portion of the ilium. From this cartilage extends dorsally into the sacral 

 area of the blastemal pelvis. Figs. 9 and 10, and there gives rise to the 

 sacral portion of the cartilagenous ilium. At the same time cartilage 

 extends into the blastema which passes anteriorl}^ to give attachment 

 to the abdominal musculature and from this arises the abdominal por- 

 tion of the cartilagenous ilium. A slight extension of cartilage into the 

 anlage of Ponpart's ligament forms the anterior superior spine, but the 

 blastemal covering of the femoral canal is not converted into cartilage as 

 is the similar covering of the obturator. 



While the cartilagenous ilium is being developed the ischial and pubic 

 cartilages extend ventrally into their corresponding blastema. The 

 ischial cartilage rapidly increases in thickness and at the same time gives 

 rise to two processes. One of these, the ischial spine, extends toward the 

 sacrum. This seems to indicate a commencing enclosure by cartilage of 

 the ilio-sciatic notch. The other projects toward the developing ham- 

 string muscles and gives rise to the ischial tuberosity. 



The pubis broadens as it extends forwards and beyond the obturator 

 foramen sends down a process which fuses with a longer one extending 

 up from the ischium. 



The various stages in the formation of a cartilagenous innominate 

 bone have been followed in detail by Petersen in a series of six embryos 

 from 17.5 to 22 mm. in length. In general what he describes coincides 

 well with the appearances presented in a somewhat more extensive corre- 

 sponding set of embryos belonging to the Mall collection. The distal 

 position of the ilium, represented in Fig. 12, Plate YII, of Petersen's 

 article, is to be looked upon as an individual variation and not as a 



