STAGE 51. Age +/- 17 d. ; length 28-36 mm.

Ext. Crit.: Tentacles much longer. *Forelimb bud oval shaped in lateral aspect*. *Hindlimb bud conical in shape, its length about 1 ½ x its breadth*; melanophores appearing on it.

Int. Crit.: Cement gland entirely disappeared; formation of unicellular glands in epidermis of hindlimb. Paraphysis with many tubules. *Cells of lateral motor columns beginning to differentiate at brachial levels of spinal cord*. 4-5 Johnson's glands on organon vomero-nasale. *Ventro-lateral lobe of saccus endolymphaticus extending forwards up to mesencephalon*. Crista parotica well developed; *formation of tectum posterius; fen. basicranalis closed; separation of for. opticum and oculomotorium*; beginning of formation of parasphenoid and frontoparietalia. *Chondrification of basal portions of atlas arches completed*. First indication of formation of secondary muscles from primary somatic musculature. *Forelimb anlage segregated into anlagen of shoulder girdle and forelimb (s.s.)*. First indication of femur anlage. Arteria iliaca communis extending to hindlimb anlage. Interrenal vein losing connection with caudal vein, the latter splitting backwards; appearance of vena femoralis. Appearance of first posterior pairs of lymph hearts. Third generation of mesonephric tubules and glomeruli; *number of augmented to 40-50*. Adipose substances appearing in cells of fat bodies. Taste buds appearing a.o. anterior to primitive tongue; filter processes of branchial arches fitting in between folds of pharyngeal roof; *folds appearing in caudal portions of lungs*; resorption vacuoles appearing in colloid of thyroid gland. *Typhlosole formed in duodenum and marginal gut*.