STAGE 52 Age +/- 21 d.; length 42-56 mm.

Ext. Crit.:*Forelimb bud irregularly conical*. *Hindlimb bud showing first indication of ankle constriction and first sign of flattening of foot*.

Int. Crit.: *Sense organs of caudal lateral line appearing on postanal fin*. *"Stirnorgan" situated just cranial to paraphysis*. Dorsal column ("dorsal horn") beginning to appear at postbrachial levels of spinal cord. *Lumbar ganglia with a few large cells*. About 10 Jacobson's glands on organon vomero-nasale.

*Separation of for. acusticum anterius and posterius*. *Chondrification completed in 4^th to 9^th vertebral arches; chondrification in lateral wall of perichordal tube extending from atlas down to intervertebral region between 2^nd and 3^rd vertebra*; 3 pairs of ribs discernable as mesenchymatous condensations. *Femur rudiment procartilaginous, tibia and fibula mesenchymatous*; local condensations of mesenchyme indicating beginning of muscle development. *Appearance of arteria subclavia*. *Beginning of sexual differentiation of gonads*.

Small pigment cells appearing in cortical region of thymus. *5 complete revolutions in outer and inner spiral of intestinal helix*.