482 COMPARATIVE ANATOMY 



Marsupials 1 (p. 35) : in the former this serves to protect the egg, 

 and in the latter the young, which are born in a very unripe con- 

 dition, thus rendering possible a longer connection between the 

 mother and embryo during lactation. The aperture of the marsu- 

 pial pouch opens anteriorly or posteriorly, according to the mode of 

 life of the animal, and is provided with a sphincter muscle capable 

 of closing it. In Marsupial embryos the margins of the lips 

 become partially fused secondarily and temporarily to form a 

 suctorial mouth, by means of which the young, many of the organs 

 of which are still in a " larval " condition, become attached to the 

 teats (cf. p. 438). 



In male Mammals, the testes arise in the same relative position 

 as the ovaries of the female. The ovary, however, does not 

 become shifted further backwards than the pelvis in the course of 

 development ; but the testis may pass out of the abdomen through 

 an inguinal canal into a purse-like outgrowth of the integument in 



Testis 



Mesorchii'ni 



Vas deferens 



Peritoneum 



Int. obi. m. 



Transversalis 



Ext. olil. 



Cremaster-sac 

 Gubeniacuhim (Lig. scroti) 



Area scroti 



FIG. 362. DIAGRAM OF THE PARTS CONCERNED IN THE DESCENT OF THE 



TESTIS. (After M. Weber. ) 



a, ligamentum testis ; b, ligamentum inguinale ; c, muscular conus inguinalis. 



the inguinal region called the scrotal sac, which is lined by a 

 continuation of the peritoneum, the tunica vaginalisY$36'2). The 

 two scrotal sacs may remain separate, or unite to form a scrotum : 

 in Marsupials this is situated in front of, and in placental Mammals 

 behind the penis. If the inguinal canals remain widely open, 

 the testes may be withdrawn periodically into the abdomen (as e.g. 

 in Rodentia and Insectivora, in which they only descend at sexual 

 maturity) : this is effected by means of the crcmastcr muscle, a 

 more suitable name for which would be the levator s. retractor testis. 

 This muscle is a continuation of the fibres of the internal oblique 

 and transversalis, or of the latter only, and corresponds to the 

 " compressor mammae " of female Marsupials. When the inguinal 

 canals become reduced (as e.g. in Man) the testes remain perma- 

 nently in the scrotum. In many Mammals, however (e.g. Mono- 



1 Traces of the marsupial folds occur in young stages of various male 

 Marsupials, and indications of a Marsupial apparatus have been described in 

 several of the higher Mammals. 



