136 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



the glands being- present on either side of the vulva as simple pyri- 

 form involutions or thickenings of the epidermis, as shown in the ac- 

 companying cut, drawn from a section passing vertically through the 

 largest portion of the mammary involution and enlarged 200 times. 



The primary epiblast, which gives rise to the epidermis ep, and the 

 deep layer of the epidermis or stratum Malpighii ep', which forms a solid 

 involution several cells deep at its fundus, is clearly the layer from 

 which in the Cetaceans, as in man, the primary acini, or mammary fol- 

 licles, are budded off. An involuted mass of cells,/, are continuous ap- 

 parently with the epidermis ep, but this mass is not sharply delimited 

 at the fundus of the involution from the stratum Malpighii, The latter, 

 however, at the mouth of the involution is quite sharply defined, as in 

 dicated in the figure, and differs in this regard from the condition of 

 aft'airs presented by a section through the mammary gland of a male 

 human foetus of five months given by Kolliker (11) ; but Huss (5) figures 

 a stage of the human mammary gland in which the Malpighian stratum 

 is almost as well defined as in my sections of the rudimentary mamma of 

 the embryo of Globiocephalus. 



The evidence is quite conclusive, so far as the development of the mam- 

 ma} of Globiocephalus afford us any insight into the mode in which these 

 structures are formed in the Cetacea, that the latter differ in no very 

 essential respect in the mode of the early development of these organs 

 from other mammals. 



While it is true that I am enabled to figure but one stage, it is un- 

 questionably a fact that that phase is approximately equivalent to the 

 five months' condition of the same organ in a human fetus. It now 

 presents the form of a simple epiblastic involution or a pyriform prolif- 

 eration of cellular elements, which have been derived, as shown by their 

 connections, from the epiblast or fetal epidermis, and this structure has 

 been gradually developed from a simple thickening at ep, which has ex- 

 tended downwards into the indifferent surrounding mesoblast m, or con- 

 nective tissue, from the superficial part of which the corium would be 

 formed at a later period. 



Of anything like buds from the lower end of this mammary involu- 

 tion, Which would represent the future acini or subdivisions of the ma- 

 ture gland, we see nothing, but that such are developed later there can 

 be but little doubt, and in a manner simulating that figured by Kolli 

 ker (11) as characteristic of the seven months' human foetus, the actual 

 terminal subdivision of the. ends of the primary acini not occurring in 

 the human species until the time of birth (Langer, 4), when they contain 

 the so-called witch's milk — Hexenmilch (D. Barfurth, 10). 



Fn one important respect the later development of the acini of the 

 mammary gland of Cetacea would doubtless differ from that of other 

 mammals, namely, in the rate at which the anterior and posterior acini 

 iiud the lateral acini wo'nld grow, the former being much longer than the 

 latter on account of the elongated, flattened form of the whole gland 



