Embryonic development of the chicken external cloaca and phallus

The use of the scanning electron microscope has provided new detailed information about the embryonic development of the chicken external cloaca and phallus and has consequently clarified the origin of the differences between the anatomy of the chicken and turkey phallus. At Day 6 (of incubation), the genital (g.) tubercle is formed in the ventral aspect of the embryo between the caudal fold of the amnion and the tail bud. On the ventral surface of the tail bud are paired g. swellings which flank the g. tubercle. Originating at the midline between the g. swellings is the g. crest which is joined cranially to the g. tubercle and later in development forms the ventral floor of the proctodeum (the most caudal cloacal compartment). By Day 8 the base of the g. tubercle has expanded caudolaterally to merge with the craniolateral extensions of the g. swellings. The g. swellings have likewise merged medially thus completing a collar-like structure around the g. tubercle. By Day 16, the dorsal and ventral lips of the cloaca have differentiated from the collar-like structure and the rounded, apical region of the g. tubercle gives rise to the primitive phallus. Further development of the external cloaca and phallus is limited primarily to the rotation of the phallus toward the ventral floor of the proctodeum and the concurrent shortening (longitudinally) of the proctodeum. Additional differentiation of the phallus occurs after hatching.