44 GENERAL EMBRYOLOGY. 
surrounded by an oval fold of skin, the genital fold, which extends from the front 
of the eminence to the root of the tail and encloses a shallow fossa, the cloacal 
fossa, at the bottom of which is the cloacal mem- 
brane. The posterior part of the cloacal fossa is 
afterwards separated from the anterior part by 
a transverse fold, the perineal fold, which crosses 
the external surface of the cloacal membrane in 
a position which corresponds internally with that 
occupied by the lower end of the septum separating 
the genito-urinary from the rectal portions of the 
cloaca. The posterior part of the cloacal fossa, 
behind the transverse fold, is the proctodeeal de- 
pression or proctodeum ; at first its long axis hes 
transversely, afterwards 1t assumes a triangular 
and then a circular form, the sphincter ani muscle 
develops in its walls, and it is transformed into the 
greater part, if not the whole, of the anal canal 
of the adult. It is separated from the rectum by 
the posterior part of the cloacal membrane, but 
when that disappears, at a date which has not yet 
been definitely ascertained, but probably about the 
third month, the anal passage forms the canal by 
which the rectum communicates with the exterior 
of the body. 
The orifices of the alimentary canal are thus 
completed. 
The Post-anal or Tail Gut.—When the hind- 
gut is first enclosed there is no tail, but a rudimentary 
tail is subsequently developed as an outgrowth from 
the dorsal end of the tail-fold, ze. from the posterior 
extremity of the body of the embryo. As the tail is 
formed, a narrow tube, which communicates in front 
with the hind-gut, is developed within it. This is 
called the post-anal or tail gut. As a rule it only 
exists for a short time, disappearing from before back- 
wards about the period when cartilage begins to be 
formed in the body and limbs, and before the cloaca 
is divided into its rectal and genito-urinary portions. 
In the few cases in which it persists it retains its con- 
Fic. 35.—DIAGRAMS showing the separa- 
tion of the cloacal part of the hind- 
gut into genito-urinary tract and 
rectum. tinuity with the rectum, which is formed from the 
A. Allantoic stalk. U. Ureter. dorsal part of the cloaca. 
B. Bladder. Ur. Urethra. 
It appears in the human subject when the embryo 
C.  Cloaca. VD. Vas deferens. pandas eae ear 
EMeicipseal membrane. "VS. Vesiculs semat- is 3 mm. long, and the rudimentary tail is just visible 
Rie Retin, WD. Walffian duct, 28 a small nodule. When the embryo attains the 
length of 4°8 mm. the anterior part of the tail-gut 
begins to degenerate, its cavity disappears, and it is converted into a solid cord of cells which 
is still attached in front to the hind-gut. In embryos 11°5 mm. long, when the tail has 
been enclosed in the posterior part of the body, the connexion of the tail-gut with the 
hind-gut is lost, and the tail-gut is represented by a small vesicle with a short cord of 
degenerating cells attached to its anterior part. 
In larger embryos the tail-gut entirely disappears. When, as in the human subject, the 
rudimentary tail is eventually embedded in the posterior end of the body, any rudiments 
of the tail-gut which persist will be found in this situation; it is stated that such rudi- 
ments occasionally develop into tumour formations. In mammals with free tails, 
rudiments of the tail-gut may be met with in any part of the tail, and apparently the 
anterior portion occasionally persists and maintains its connexion with the rectum, from 
which it extends backwards as a narrow and blind diverticulum. 
