57. On the Pelvis of the Anura: a Study in Adaptation and Recapitulation.



A. Development of Pelvic Region.

Pelvic Cartilages.

  • 1The pro-cartilaginous pelvis first appears in tadpoles of about 27 mm. as widely separated elements, each of which represents half the girdle.
  • 2At 30 mm. the girdle is chondrified, the halves are still widely separated and be at a wide angle to each other. The acetabulum opens backwards.
  • 3There is evidence for the presence of two centers of chondrification in each half-girdle, instead of one, as hitherto considered.
  • 4The pars pubis probably has no separate center of chondrihation, but arises from the lower center, which forms a wide puboischiadic plate.
  • 5The absence of an ossified pubis in Phaneroglossa is thus probably correlated with the absence of a centre of chondrification for this region.
  • 6At 33 mm. chondrification is completed, the pelvic halves approach, and the angle between them is reduced.
  • 7At 34.5 mm. the ischiadic regions are nearly in contact and the acetabulum lateral.
  • 8At 33 mm. p.m the ischiadic symphysis has begun, together with the rotation of the pelvis to a very slight extent.
  • 9At, 31 mm. p.m. the symphysis is increased whilst pelvic closure is completed.
  • 10At 20 mm. p.m., rotation is very marked, together with much iliac. growth. The ilio-sacral connection is well developed. The acetabulum opens laterally.
  • 11At 14 mm., a newly metamorphosed frog, the pelvis differs from the mature adult only in being cartilaginous.Muscles.
  • 12The limb-buds arise early, and are first filled with undifferentiated mesenchyme into which blood-vessels, and then nerves penetrate.
  • 13Myotomic derivatives probably enter the limb-buds at an early stage.
  • 14Differentiation into dorsal and ventral masses begins at 20 mm.
  • 15Many separate muscles and muscle-groups are present at 30 mm.
  • 16The muscles grow from insertion to origin, and the latter may often be late in gaining definition.
  • 17At 34–5 mm. all the muscles of the adult (except Gaupp's derivatives of Ecker's M. Adductor brevis) are present.
  • 18The relations of the muscle origins show various changes during development.
  • 19Development shows that M. Adductor magnus originates from the fusion of two muscles, a conclusion previously dependent upon comparative anatomy.
  • 20M. Semitendinosus forms its origins and reaches its deep position late in development, supporting the theory that it reached this position phylo-genetically late in the Ranidæ.
  • 21M. Gracilis minor arises as a slip of M. Gracilis major.
  • 22M. Pyriformis differentiates early, but its origin is not defined until relatively late, its first attachment being to a mesenchymatous mass below the notochord.
  • 23The urostyle does not completely chondrify until 22 mm. p.m., when the M. pyriformis attaches thereto.
  • 24The “primary abdominal muscle” preceding M. Rectus abdominis at first passes through the pelvic basin to a point posterior to this. During the closure of the pelvis it moves forwards and becomes attached to the pubic symphysis.Nerves.
  • 25The early limb-bud is innervated by one spinal trunk, which is closely followed by two others.
  • 26At 27 mm. there is a pelvic plexus formed of three spinal nerves.
  • 27N. coccygeus, the tenth spinal nerve of the adult, does not anastomose with the plexus until metamorphosis.