138 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



muscles, while the calf has its mouth to the nipple, for only short inter 

 vals between the times when it rises to the surface for air. 



The peculiar position of the nipple, which is sunken into a longitudi- 

 nal fossa which quite covers or incloses the former, is a characteristic 

 feature of the external conformation of these organs in the Cetacea, and 

 may be regarded as a physiological adaptation similar in nature to the 

 extension and flattening of the gland itself, as a result of which the 

 milk-secreting organs do not bulge outwards as in other mammals, but 

 helps the animal to retain its normal fusiform shape, with no portion 

 of these secondary sexual organs projecting outwardly, thus protecting 

 them from injury and not impeding the movement of the parent through 

 the water, as suggested by Owen. 



The question now arises, to which category the mammary gland of 

 Cetaceans must be assigned, namely, those with true nipples, such as 

 are found in the Carnivora, or those with pseudo-nipples, such as are 

 found in Ungulata, certain Marsupialia, and Murina. To judge from the 

 structure of the adult nipple, with its single opening, there can, I think, 

 be little doubt of the propriety of classing the mammary gland of Ce- 

 tacea with that group which has been characterized by Gegenbaur (fi) 

 as possessing pseudo-nipples, which are developed by the production 

 of the margin of the primary mammary area of the embryo into a tubu- 

 lar prolongation, and which in all cases is characterized by the pos- 

 session of a single external opening, as in the cow. By what process of 

 development, however, the great median sinus of the Cetacean mam- 

 mary gland was produced we do not know, and must wait for the eluci- 

 dation of this part of the subject through the study of more material. 



Turner (12) speaks of the sinus as being lined by a mucous membrane, 

 but the question arises, has this endothelial lining of the gland arisen 

 by involutions from the primary epiblastic involution, or has it arisen 

 partly by vacuolization and retrogressive histological processes, as ar- 

 gued by Creighton (7) and Rein (9). To me it seems probable that both 

 processes, as shown by the last-named investigator, are involved, namely, 

 those of involution or proliferation from the primary gland bud, and 

 vacuolization, which latter process probably steps in later, or after the 

 foundations of the principal acini have been laid down by the first pro- 

 cess. 



In some forms it would seem probable that the mammary glands al- 

 most wholly disappear during the intervals between the periods of ges- 

 tation, as observed by Allen (8) in the bats, to be regenerated again 

 beneath the integument with the progress of the period of gestation. 

 Facts such as these would seem to favor the opinions and suggestions 

 put forward in 7, though there can be no doubt whatever as to the 

 fact that the first traces of these organs exist as thickenings or prolif- 

 erations at definite regions in the epiblast, and that eventually such a 

 thickening shoves the stratum Malpu/Jiii, downward before it a$ a pretty 



