Farre^ on an Early Human Embryo. 
167 
viewed in profile^ appear single^ but which occur in pairs. 
These constitute no permanent parts or organs^ but become 
transformed into various structures, such as the maxilla, 
mandible, &c. They are foetal peculiarities of a most in- 
teresting class, forming here the visceral arches, or visceral- 
bogen of German physiologists, while the fissures or apertures 
between them are known as the branchial fissures or apertures. 
They are one stage less advanced in the present instance than 
they are in the embryo which I formerly described. Op- 
posite to the lowermost pair of leaflets, a looped blood-vessel 
enters the trunk. This leads from the heart, at this period 
consisting of three chambers only, viz., a single auricle and 
ventricle, and a bulbus arteriosus. In my former paper 
having more particularly traced the development of the heart, 
and examined its composition, both at this stage of embryonic 
life and also at periods somewhat later, I refrain here from 
any further remarks upon this interesting portion of the 
subject. 
Nearly on a level with the heart, though a little lower 
down, will be perceived a slender lamina projecting from the 
side of the trunk in the form of a small fin or paddle. This 
is the sole rudiment at this stage of an arm or anterior ex- 
tremity. It constitutes the portion of blastema in which is 
subsequently laid down the chief portions of the structures 
forming the hand and arm, but which is not more developed 
at this stage of embryonic life than is the paddle of the whale 
or porpoise. 
On the other hand, the rudiment of the lower limb or 
posterior extremity is more easily distinguished, in the form 
of a bud, projecting from either side of the trunk near its 
termination, but, like the anterior extremity, as yet in so 
rudimental a form that the last vertebrae of the spinal column 
still project beyond it. 
Emerging from between these is the root of what I presume 
to have been the allantois, but which was unfortunately cut 
across in the manner already explained. By this channel the 
blood, after circulating through the body of the embryo, would 
be carried to the membranes by which it is encompassed, and 
at this point doubtless the connection between the parent and 
offspring, such as it is at this period, is established. 
In front of the spinal column are distinguishable two or 
three structures, one of which only is sufficiently definite 
to admit of precise delineation. It is in the form of an 
elongated body, having apparently a duct running downwards 
towards the root of insertion of what I suppose to be the 
allantois. I have little doubt that this is a rudimental 
