28 COMPARATIVE ANATOMY. 



case the teat may be described as true or secondary (Mar- 

 supials, Rodents (? all), Lemurs, Monkeys, and Man), and in 

 the former as a pseudo- or primary teat (Garni vora, Pigs, 

 Horses, and Ruminants). The lattej condition is already 

 sketched out in certain Marsupials (Phalangista vulpina). 



As a rule the number of teats corresponds to the number of 

 young born at a time. They are often situated in two nearly 

 parallel rows along the ventral side of the thorax and abdomen, 

 which gradually converge towards the inguinal region : in other 

 cases they may be restricted either to the inguinal (Ungulates and 

 Cetaceans) or to the thoracic region (Elephants, Sirenia, many 

 Lemurs, Cheiroptera, and Primates). 



In the human male, the mammary apparatus becomes aborted, though 

 usually at birth and puberty true milk, the so-called " witches' milk " 

 (Hexenmilch), is produced. Male goats and castrated sheep have also been 

 known to give milk. The occasional existence in men of supernumerary teats, 

 and in women of supernumerary mammae and teats (poly mast ism and poly- 

 thelism) is very remarkable. They are usually situated in the thoracic region, 

 and must be considered as an atavism to a characteristic primitive form which 

 possessed numerous teats, and which produced a number of young at a time. 

 Such a transition from polymastism to bimastism may be seen plainly at the 

 present day in the Lemurs : in them the inguinal and abdominal teats are under- 

 going a retrogressive metamorphosis, while a single pair of thoracic teats remain 

 well developed. 1 This accords with the fact that most Lemurs bear only a pair 

 of young ones at a time, which they carry with them at the breast. 



The glands, which are at first solid, become secondarily hollowed 

 out and differentiated into acini, mammary ducts, milk 

 sinuses, and excretory ducts. The whole intermediate tissue 

 during lactation is filled with white blood-corpuscles (leucocytes), 

 and possibly the well-known structural elements of milk, known as 

 colostrums and milk-spheres, owe their origin to these corpuscles, 

 which have passed through the walls of the acini. 



BlBLIOGKAPHY. 



CARRIERE, J. Die postembryonale Entwickelung der Epidermis des Siredon pisci- 



formis. Archivf. mikros. Anat. Bd. XXIV. 1884. 

 DOMBROWSKI, R. VON. Geweihe u. Gehorne. Nalurwiss. Studic. Wien, 1885. (With 



40 plates.) 

 ECKEH, A., und WIEDERSHEIM, R. Die Anatomic dcs Frosehcs. Braunschweig, 



186482. 

 GEGENBAUR, C. Zur Morphol. dcs Nagels. MorpJi. Jahrbuch, Bd. X. 1885. Zur 



genaueren Kenntniss der Zitzen der Sdugethiere. Morphol. Jahrb. Bd. I. 1876. 

 HUXLEY, T. H. Tegumentary Organs. Todd's Cyclopaedia of Anat. and Physiol. 

 KERBERT, C. Ueber die Haut der Reptilian und andrer Wirbelthivre. Arch. f. 



miTcr. Anatomie, Bd. XIII. 

 KLAATSCH, H. Zur Morphologic der Sdugethierzitzen. Morphol. Jahrb. Bd. IX. 



1883. 



LEICHTENSTERN, Ueber uberzdhlige Brustc. Arch. f. pathol. Anat. 1878. 

 LEYDIG, F. Ueber die allgem. Bedeckungen der Amphibien. Arch. f. mikr. 



Anatomie, Bd. XII. 1876. 

 LIST, J. Ueber Wanderzellen in Epithcl. Archiv /. mikros. Anat. Bd. XV. 1885, 



mid Zool. Anzeiger, No. 198, VII. Jalirgang, 1885. 



1 In Hapalemur griseus the single pair of teats is situated on the arm (Becldard)-. 



