REPRODUCTIVE HISTORY OF THE MOLE, 195 
knowledge of the process of development of the external genitalia 
had been limited to the detailed study of human embryos. I had 
long since satisfied myself as to the method of development of the 
human external genitalia, and I had become convinced that some 
of the processes which are usually described did not in fact exist. 
The most cursory examination of my series of mole embryos, 
however, seemed to show that what I had regarded as, and stated 
to be (6), inaccuracies in the account of human development, were 
actual and obvious phases in the mole. Such a result necessi- 
tated a most careful review of my material, for it seemed that 
what was so obvious in the mole embryo was sufficient to stultify 
my assertions regarding human development. ‘To assure myself 
of the correctness of my interpretation of the phases in the mole 
was therefore essential, and I have accordingly submitted all my 
material to a very critical examination. The results given here 
may therefore be taken as those which are so evidently true 
that, after repeated examination, they had to be accepted—though 
unwillingly—as correct. 
The reinvestigation of the phases in human development was 
the next step, and the results of this part of the research I reserve 
for a further paper. 
In the mole embryo measuring 9 mm. along the dorsal surface 
from the vertex to the caudal bend, the rudiments of all the 
essential elements of the external genitalia are readily recognisable. 
Such a stage is shown in text-fig. 1(A & B). The cloaca is present 
as a wide orifice, from the ventral margin of which the genital 
tubercle projects. The urethral groove is present upon the cloacal 
aspect of the genital tubercle, and the,margins of the groove (inner 
genital folds) are widely separated at the base of the tubercle and 
fade away posteriorly within the cloacal margins. The separate 
visceral orifices opening into the cloaca are not recognisable by 
external examination at this stage. Upon the lateral aspects of 
the base of the genital tubercle, continued in a caudal direction 
as the lips of the cloacal opening, are the labia majora (labio- 
scrotal folds). The labio-scrotal folds are lost in a cephalic 
direction upon the skin of the abdomen in front of the genital 
tubercle ; they swell out as two ovoid prominences at its base, 
and diminish again towards the tail by skirting, as slightly 
elevated margins, around the cloacal orifice. 
In the next stage, an embryo of 18 mm. shows that the genital 
tubercle has elongated, and the urethral groove become closed by 
the meeting of the labia minora along its cloacal aspect; but the 
outstanding feature of the changes consists of an extensive growth 
of the labio-scrotal folds (text-fig. 1,C & D). So extensive is this 
growth that the base of the genital tubercle has been buried 
beneath the ingrowing mesial margins of the two ovoid promi- 
nences present in this position in the 9 mm. stage. The urethral 
channel has been completed by closure of the inner genital folds, 
and now, over this closed urethra, the outer genital folds have 
met in the mid line. The dorsal portion of the cloaca is now 
13* 
