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

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