1885.] Embryology. 517 
digital border. 2. The existence of a small median papilla, ac- 
cording to Wilder, at the extreme end and under side of the tail 
of the foetus of the manatee, representing apparently the last 
vestige of an exserted tail extending beyond and behind the fluke © 
folds of this type, in which the flukes are in fact rudimentary. 3. 
The fact that the osseous elements of the limb have atrophied ex- 
actly in the reverse order in which they appear in the embryo, or 
from without inwards, that is, from behind forwards in cetaceans 
and sirenians, because in both, the hind-limbs have been rotated 
or extended permanently backwards distad of the knee-joint. 4. 
The structure of the embryonic fluke-folds or diverticula filled 
with mesoblast comparable to that found in the limb-folds of 
other vertebrate embryos, these limb-buds representing structures 
which have survived translocation and made an attempt to im- 
perfectly recapitulate the development of part of the limb. - 
The above headings present the embryological argument. The 
other data are anatomical and are mainly based on a comparison 
of the pinniped and cetacean types. Admitted that the cetaceans 
are descended from land forms, we would naturally look to types 
of amphibious habits and poorly adapted for progression upon 
land to furnish the first indications of modifications which have 
been carried to an extreme degree in the former. Traces of the 
beginnings of such modifications we actually find in pinnipeds. 
In the pinnipeds, the hind limbs, from the knees, have been 
rotated backward and included by a continuation of the integu- 
ment which invests the body together with the tail, leaving only 
the last two or three short caudal vertebræ exserted or projecting 
into a caudal integumentary pocket, lying between the distal parts 
of the backwardly extended limbs.. This process of inclusion, if 
carried to an extreme stage, would finally cause the whole of the 
tail to be lost to sight outwardly, leaving only the metapodial and 
phalangeal parts free, As a result of this arrangement in the 
pinnipeds certain muscular insertions of the limb muscles have 
been moved backwards, and the hyposkeletal flexors of the trunk 
have become more powerful; the abdominal muscles extending 
over the knees have restricted the movements of the femur.- As 
a further result of this restricted movement the pelvis has 
to degenerate, the symphysis pubis become less defined, and the 
femur shortened. We are therefore, I submit, the actual witnesses 
of a process in the pinnipeds which if carried still further would 
bring about the condition now found in living cetaceans. The 
pedes in pinnipeds have been hypertrophied together with the 
metapodial and phalangeal elements, but are not the fingers also 
lengthened and their joints multiplied in the cetacean manus ? 
In the Plesiosauri, Ichthyosauri and Lyrifera or true fishes, the 
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or increase of limb-elements comparable to p langes, with a 
corresponding shortening of the proximal bones in contact with 
