THE HIND-GUT AND ANAL PASSAGE. 43 
The tympanic cavity is developed from the dorsal or upper end of this portion 
of the cleft, and it is prolonged upwards on the outer side of the otic vesicle which 
simultaneously descends in the tissues of the head. Thus the upper end of the 
inner portion of the cleft, which is somewhat dilated, comes to lie between the otic 
vesicle, which is developed into the internal ear, on the inner side, and the tympanic 
membrane which separates it from the external auditory meatus on the outer side, 
and it remains in the adult as a laterally compressed space, the tympanic cavity, 
which is continuous through the Eustachian tube with the upper part of the 
pharynx. In the mesoderm round the i inner, upper, and back part of the cavity the 
petrous part of the temporal bone is developed and ossified, and in the lower and 
anterior part the tympanic ossification extends outwards during the formation of 
the tympanic plate. 
The upper part of the tympanic space is prolonged backwards between the ossifying 
petrous and squamous parts of the temporal bone, where it forms a recess known in the 
adult as the mastoid antrum, from which at a later period diverticula are projected into 
the mastoid portion of the temporal bone, forming the mastoid air cells. 
The lower portion of the inner part of the cleft is moved obliquely forwards. 
As development proceeds it is contracted and carried downwards and forwards in 
front of the developing otic vesicle. It is the rudiment of the Eustachian tube, 
and, as the septum which separates the nasal chambers from the mouth is formed, 
its lower end attains a position just behind and at the side of the posterior narial 
orifice in the upper and lateral part of the pharynx. Apparently, therefore, the 
lower end of the Eustachian tube has a much higher position than that originally 
occupied by the lower end of the cleft from which it is formed, for it will be 
remembered that the lower end of the first visceral cleft is situated, in the early 
stages, at the side of the tuberculuin impar from which the anterior two-thirds of 
the tongue is formed. This alteration in relative position is due, however, not to 
elevation of the lower end of the first visceral cleft during its transformation into 
the Eustachian passage, but to the enormous downgrowth of the mandibular arches, 
which carry with them the tongue, as they enlarge to form the lower jaw. 
THE HIND-GUT, THE ANAL PASSAGE, AND THE POST-ANAL 
OR TAIL GUT 
By the formation of the mouth the primitive alimentary canal opens anteriorly ; 
it remains closed posteriorly until a later date, when the anal passage and orifice 
are developed. 
The posterior end of the hind-gut which is enclosed in the tail-fold is termed 
the cloaca. The cloaca is dilated, and, assuming a conical form, receives the 
terminations of the genito-uriary ducts. It is bounded postero- inferiorly by the 
cloacal membrane which extends from the root of the tail to the body stalk by which 
the embryo is attached to the chorion. The cloacal membrane is modified from the 
posterior part of the primitive streak ; this remains on the surface of the body after 
the anterior part has been separated and enclosed during the completion of the 
posterior part of the neural canal, and it forms a septum between the cavity of the 
cloaca and the exterior. It consists at first of ectoderm and entoderm alone, and 
it is only at its lower and anterior part that it is subsequently invaded to a slight 
extent by mesoderm. 
During the second month of intrauterine life the cloaca is divided into a ventral 
or genito-urinary, and a dorsal or rectal section, by the formation and fusion of 
lateral folds, which gradually unite, from before backwards, till finally the posterior 
-end of the septum approaches and fuses with the cloacal membrane, and the 
rectum is separated from the genito-urinary chamber. Before this separation 
is completed an eminence appears in the region of the anterior part of the 
cloacal membrane at the junction of the ventral surface with the posterior 
extremity of the body, 7.e. in that part which afterwards becomes the region of the 
symphysis pubis. This eminence is the genital eminence, and from it are formed 
the penis in the male and the clitoris in the female. The genital eminence is 
all 
* 
