140 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



In a male foetus of Delphinus Bairdii 11 inches long, belonging to the 

 National Museum collection, no traces of mammary fossae could be found. 

 In another female foetus in the collection, identified with some doubt 

 by Mr. True asPhocwna lineata, and 17^ inches long, the mammary clefts 

 were 8 millimeters loug, the nipples distinctly developed and about 1.5 

 millimeters in diameter. 



In a male foetus 33£ inches loDg, referred to Balcenoptera musculus, 

 (No. 13763) by Mr. True, the rudimentary mammary clefts are about 

 one eigth of an inch long, and are situated about half way between the 

 anal opening and the proximal part of the preputial membrane covering 

 the base of the penis behind, and seven-eighths of an inch in advance of 

 the anus. The nipples are not nearly so well developed in proportion 

 as in the female foetus of Phoccena 17 J inches long, being only .75 milli 

 meters in their longest and .5 millimeters in their shortest or trans- 

 verse diameter. The pair of mammary clefts and nipples of opposite 

 sides in this specimen were situated about one- fourth of an inch apart, 

 measured across the middle line of the foetus. 



It is therefore evident that there is an important difference existing 

 between the foetal males of some of the species of Denticete and some of 

 the Mysticetc, inasmuch as the latter possess rudimentary mammary 

 glands and the former do not. How universally this may be true we 

 will not know until foetuses of all the farms have been studied. 



The sexes of young Cetaceans are already distinguishable when they 

 are about 2 inches long, and when 3 inches long the genital raphe 

 has closed in the male and the perineum is already much longer than in 

 the female foetus; it is, in fact, more than twice as long in a male 3 

 inches loug than in a female foetus 2 inches in length. It is therefore 

 evident that the indifferent stage development of the external genitalia 

 of the embryos of Cetaceans must be passed over before they have 

 reached the length of 2 inches, so that the dimensions reached when the 

 sexes become differentiated externally probably correspond pretty 

 closely with those of the embryos of the human species. 



Wyinan, in examining an embryo of the great right whale, Balceua mys- 

 ticetus* found it difficult to discriminate the sex of it externally, although 

 6 inches in length. In the series of embryos in my possession there does 

 not seem to be any difficulty of this sort, as the single male specimen of 

 Phocama, with its closed raphe between the penis and anus, at once gives 

 the unmistakable outward indication of the sex to which the specimen 

 belongs. 



This was an important matter to determine in order to discriminate 

 the specimens used in the preceding discussion of the development of 

 the mammary gland. 



The results arrived at in the foregoing paragraphs may be summar- 

 ized as follows : 



1. The mammary gland of Cetaceans develops from a thickening of 



* Proc. Bost. Soc. Nat, Hist., ii:, 1848-'51, p. 355. 



