198 THE STRUCTURE OF MAN 
testis), (2) evaginated derivatives of the internal oblique and 
transversus muscles (cremaster), and its cavity is connected with 
the celom by a special canal (canalis vaginalis in the male, 
canalis Nuckii in the female). 
The differentiation of these parts, which was in all probability 
originally effected only in the adult, in some cases takes place at 
an earlier (Mouse) or even embryonic period (Squirrel). _ | 
It is conceivable that next in order to the type represented 
by Rodents and Insectivores, there may: have existed forms 
in which the descensus occurred periodically in youth, but in 
which, in more advanced age, in consequence of the loss of the 
reditus testium at the rutting season, it became fixed. Such 
forms are not actually known; but the hypothetical stage is very 
nearly realised in Man, as in him, by the partial reinvagination 
of the bursa, and by the consequent formation of a conus 
inguinalis, we are still reminded, ontogenetically, of the periodical 
descensus and reditus testium, although it 1s but a very feeble 
process. There is thus reason for thinking that, among the 
Prosimi and Primates, forms corresponding with this hypotheti- 
cal stage might be found. 
The definitive descensus is due to a further evagination 
of the conus. The bursa inguinalis, however, which was once 
(as in the Rodents and Insectivora) the direct product of this 
very shifting of the testis, in Man first arises independently at 
some distance from it, forming what is known as the genital 
ridge or the outer genital fold. 
Among the lower Mammals the development of a permanent 
scrotum has become established in the Marsupialia, Ungulata, and 
Carnivora. Among the LEdentata only the Orycteropodide 
possess a testis sac into which the testes periodically enter. In 
Dasypus, Bradypus, and Myrmecophaga the testes are abdominal ; 
in Manis they are subintegumental, and le in the inguinal region. 
In the Monotremes a descensus testiculi 1s not known to occur. 
In considering the phylogenetic origin of the descensus testiculorum, 
Klaatsch has formulated the following ingenious argument :—-The mammary 
organ, which in the form of a somewhat circular patch of the integument, 
characterised by glands and smooth musculature, first became differenti- 
ated in the inguinal region, exercised a great influence on the abdominal 
wall. He has suggested that among the ancestors of the Mammals there 
occurred, as he believes is shown by the Monotremata, a transference of the 
mammary organ from the female to the male ;! and that this may have 
1 In other words, Klaatsch interprets as the homologue of this Mammary area a 
circumscribed wrinkled portion of the integument, only scantily covered with hair, 
———! . 
