1885.] Embryology. 617 
to give my results together with such other information as could 
be gathered from the examination externally of the mammæ ofa 
female whale’s fœtus, five and a-half inches long, belonging to the 
Pacific genus Rhachianectes. The stage here figured displays 
the gland in the undifferentiated condition of the five-months’ 
human embryo, when the gland consists merely of an involution of 
the malpighian layer, ef’, filled by a solid core of more rounded 
cells, f which seem to become blended, at the lower end of the 
involution, with the Malpighian layer, the whole structure present- 
ing the appearance of a solid pyriform body jutting down into 
the mesoblast, m, and connected with the epidermis externally by 
a narrow pedicel. 
No signs of the outgrowth of the rudiments of acini from this 
pyriform body have yet appeared, but it would be inferred from 
the shape of the gland in the adults that these acini would be . 
most apt to first appear at the anterior and posterior sides of this 
body. The gland in the adult cetaceans is greatly elongated, flat 
and less than one-third as wide as long, reaching the enormous 
dimensions of ten feet in length, three feet in width and eight 
inches in thickness in the adult, gravid female of Balaenoptera 
sibbaldit. In the adult the gland is also traversed longitudinally 
by a spacious lacteal sinus, which is probably developed during 
the growth of the gland by a process of vacuolization. This sinus 
opens externally through the nipple by way of a single duct. The 
gland therefore probably belongs to that subdivision of mam- 
mary organs provided with pseudo-nipples, which are developed 
by the production of the edge of the embryonic mammary area 
into a tubular teat traversed by a single canal as in the cow, cer- 
. tain marsupials and rodents. 
In combination with the peculiar internal structure of the mam- 
mary gland of cetaceans, there is also an external teleological 
which they must be over a foot in length. 
It thus becomes evident that the mammary glands of cetaceans 
