T12 BULLETIN OF THE 
are developed in the intermediate cell-mass, which previously appeared 
as a continuous solid mass of cells, a series of circular cavities, each 
opposite a protovertebra. The cavities thus formed in the intermedi- 
ate cell-mass are the “segmental vesicles ” described by Rathke (733) 
and later observers. Weldon has shown that the segmental vesi- 
cles are not evaginations of the peritoneal epithelium, and do not 
form the Wolffian duct, as supposed by Braun, but that the latter 
structure is developed a little later and in a more peripheral or lateral 
region of the intermediate cell-mass,— indeed, from the lateral walls 
of the segmental vesicles. Other observers, however, differ in their 
views concerning the details in the formation of the Wolffian duct, 
and I do not propose to enter upon the discussion of this interesting 
subject. It is sufficient for my purpose to recognize the presence of 
the intermediate cell-mass in Lacertilia, and to note the fact that in 
its origin and fate it resembles very strongly the intermediate cell-mass 
in Birds and Elasmobranchs. 
In Teleosts there is also present a structure very similar to the inter- 
mediate cell-mass of Birds, Reptiles, and Elasmobranchs, whose homology 
with the intermediate cell-mass has, however, been questioned by Ziegler 
(’87, p. 645). Oellacher (’73, pp. 76, 77, 102) was the first to describe 
this structure in Bony Fishes, and to adopt for it the name “ intermedi- 
ate cell-mass.” According to his observations, in the Brook Trout, at 
the stage in which the vertebre are forming, there remains between the 
lateral plates and protovertebree a band of undifferentiated mesoderm, 
which later moves medianward, and meets its fellow from the opposite 
side, uniting with it under the chorda. Ziegler (’82 and 87) and 
Wenckebach (’84 and ’86) have since observed this structure in the 
Salmon, Trout, and Pike. The intermediate cell-mass in Salmo salar 
is formed, according to Ziegler’s (’82) account, at the time when the 
protovertebre are cut off, which takes place on the thirteenth day, and 
after this process it remains as an unsegmented band of cells, extending 
throughout the whole length of the trunk of the embryo, and embracing 
the entire thickness of the mesoderm. It soon becomes crowded ven- 
trally and medianward, so that the lateral layers draw near to the pro- 
tovertebre. The lateral band, constituting the intermediate cell-mass 
of either side of the body, moves toward the median plane, and the two 
bands finally meet above the intestine, where they form a single com- 
pact mass or rod of cells, which in an embryo of nineteen days extends 
from the pronephros to the posterior end of the trunk. Ziegler (’82, 
pp. 46, 47) in his description implies, without directly stating it as a 
