198 THE STRUCTURE OF MAN 



testis), (2) evaginated derivatives of the internal oblique and 

 trans versus muscles (cremaster), and its cavity is connected with 

 the coeloni 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 Eodents 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 is but a very feeble 

 process. There is thus reason for thinking that, among the 

 Prosimii 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 Eodents 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 Edentata only the Orycteropodidse 

 possess a testis sac into which the testes periodically enter. In 

 Dasypus, Bradypus, and Myrmecopliaga, the testes are abdominal : 

 in Manis they are subintegumental, and lie in the inguinal region. 

 In the Monotremes a descensus testiculi is 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 ; x 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, 



