BULLETIN OF THE UNITED STATES FISH COMMISSION. 139 



well defined layer into the mesoblast, as shown in the accompanying 

 ft £n re. 



The development of the dermal folds which inclose the nipple of 

 Cetaceans, however, complicates the development of the nipple itself 

 somewhat, so that it is probably impossible at present to reach any 

 very positive conclusions, unless we may assume that the stages of the 

 development represented by an embryo of Olobiocephalus melas 2 inches 

 long and one of Bhachianectes glaucus 5£ inches long will serve to throw 

 some light upon this question. 



The sections which were made of the young of Globiocejphalua of the 

 size mentioned show that the folds which form the wall of the nipple 

 fossa were only in their incipiency or scarcely at all developed during 

 the 2-inch stage, as shown in the second diagrammatic figure, taken 

 from a section at a somewhat different level from the upper more de 

 tailed one, this one striking nearly the center of the incipient nipple, 

 which seems to be present as a slight elevation of the epidermis, while 

 on either side of it there are two swellings, r r, which seem to me to 

 represent the developing side- walls of the nipple fossa. 



What convinces me that the preceding opinion is justified is the ex- 

 ternal appearance of the nipple fossae or clefts in the more advaueed, 

 female embryo of Rkachianectes, 5% inches long. In this specimen the 

 mammary clefts, without externally visible indications of the nipples, 

 are already formed as a pair of minute longitudinal slit-like depressions, 

 which, without much doubt, represent the proportionally large clefts 

 which open from without into the nipple fossa in the adult. In this 

 last stage these clefts are about .5 millimeter in length, and, on account 

 of the greater proportional size of the clitoris of the embryo, are placed 

 relatively much closer to that organ than in the adult. Were it pos 

 sible to investigate the condition of the mammary gland in this larger 

 embryo, it might be that light would be thrown upon the steps by which 

 the gland itself is formed; but as the specimen is a unique one, having 

 been figured by Scammon, and unfortunately belongs to a Pacific species 

 which I am told by Mr. Dall is rapidly approaching extinction, great 

 hesitancy has naturally been felt by the Museum authorities as to the 

 desirability of sacrificing it for purposes of anatomical investigation. 



There seems to me, therefore, to be but little doubt remaining that the 

 nipple fossa of Cetaceans is developed during a comparatively early 

 stage, or in the interval in the history of the intrauterine growth of the 

 young Cetacean corresponding to that between the fifth and seventh 

 months of the human foetus. 



Turner (12) has described rudimentary mammary fossae, behind and 

 a little to either side of the base of the penis of an advanced male 

 foetus of Balccnojrtera Sibbaldii 18 feet long, but in the much younger 

 stages of development of Phocccna communis, represented by a male fee 

 tus 3 inches long now in my hands, I cannot find any evidence of such 

 rudimentary mammarv clefts or fossae as are described by Turner. 



