1885.] Embryology. IO15 
absence of elements normally formed is concerned. I am there- 
fore constrained to believe that it has been through a seal-like 
ancestry with prolonged integuments to the manus, in which the 
nails were not terminal but dorsal, beyond which the ungual pha- 
langes were extended as bars of cartilage, which gave rise, by 
transverse segmentation and subsequent ossification to extra ter- 
minal digital segments as found in existing Cetacea. 
The second fact of importance to be considered as lending 
probability to the foregoing view is the circumstance that when 
a limb is in its primitive cartilaginous condition it always devel- 
ops its segments from its axial end toward its peripheral end in 
serial order. The basis for the extra terminal segments was first 
developed through the influence of functional adaptation, as car- 
tilaginous bars or extensions of the primitive cartilage of the 
ungual elements of the digits in response to the demands made 
upon the limb in swimming. The segmentation of these terminal 
cartilaginous bars then followed through the influence of mechan- 
ical strains acting upon the cartilaginous terminal bars as these 
were alternately bent in opposite directions. These conclusions 
might be still further illustrated by data obtained from the obser- 
vation of the development of the limbs of other animals. 
These views, very forcibly it seems to me, sustain the hypoth- 
esis that the Cetacea are the off-shoots of land forms which were 
at first terrestrial, or at least amphibious, as are the pinnipeds. 
Such views do not at least run counter to any morphological 
facts, but in reality are sustained by them.— Fohn A. Ryder. 
ON THE MANNER IN WHICH THE CAVITY OF THE HEART IS 
FORMED IN CERTAIN TELEOsTS.— Balfour (Comp. Embryol., 1, 
523) states that “ in Teleostei the heart is formed as in birds and 
mammals by the coalescence of two tubes, and it arises before 
the formation of the throat.” I would now point out that this is 
not universally true of all teleostean forms, in fact it seems not 
to be the mode of cardiac development in any species of this 
group so far investigated. 
My own observations on this point have been made upon the 
embryo cod-fish and indicate very conclusively, so far as trans- 
parent views are of value in deciding such a difficult matter, that 
the lumen of the heart arises asa single and not as a double 
cavity which afterwards blends to form a single one, as in birds 
and Mammalia. Moreover, this cavity is from the first open 
below, and is present in the cod embryo, on the fifteenth day, as 
a round opening limited all round by mesoblastic cells. Its 
transverse diameter at this stage is .043™™ and is probably half as 
cep. It has the appearance of a round perforation in the meso- 
blastic plate of cells which probably gives rise to the endothel- 
lum of the cardiac cavity, the lining of the pericardial cavity and 
* Contrib. Embryog. Oss. Fishes, pp. 82-86. 
