166 
FarrEj on an Early Human Embryo. 
These great nervous centres appear at first in the form of 
vesicles^ arranged in a linear series^ whose unequal distension 
of the thin textures by which they are at this period covered 
in and protected causes the bulgings or tuberosities^ so con- 
spicuous in the part representing the head of the embryo. 
Three of these swellings may be readily distinguished. The 
most anterior of these (b) indicates the seat of the double 
vesicle which is afterwards developed into the cerebral hemi- 
spheres_, at this time known as the prosencephalon. Its small 
relative size at this early period of embryo life is an interesting 
circumstance^ when we consider how closely the cerebral hemi- 
spheres are connected with the intellectual faculties^ and how 
greatly these portions of the brain preponderate in the fully 
developed organ. As^ however^ these faculties are not called 
into play at an early period of life^ while^ on the other hand^ 
the organs of sense and relation must be exercised imme- 
diately upon the act of birth^ we are not surprised to find 
the relative amounts of development of the nervous centres 
corresponding with these requirements. Behind the seat of 
the prosencepalon is observed another and rather larger 
bulging (c) , which marks the situation of the mesencephalon 
or single vesicle of the corpora quadrigemina. Within the 
same bulging, but a little more anteriorly, is also found the 
vesicle of the third ventricle and the optic thalami. The 
mesencephalon, it will be noticed, is here larger than the 
prosencephalon, and, considering this as the great ganglion of 
the optic nerves, its great proportionate size is illustrative of 
the large relative proportion which this part of the brain 
exhibits in the lowest vertebrata. The third swelling (n) in- 
dicates the seat of origin of the acoustic nerve. It is the 
epencephalon, and contains the vesicle from which are deve- 
loped the cerebellum and medulla oblongata. 
Beyond this point a slight bending outwards indicates the 
situation of the commencement of the spinal cord, while the 
trunk is continued flexuous, but of nearly uniform size^ until 
it terminates, after an abrupt curve forwards, in a pointed 
prominence indicating the seat of the future os coccygis. 
Already it may be seen that the vertebrse are marked out^ 
and, by the aid of transmitted light, their little quadrate 
divisions may be very distinctly discerned in the semi- 
translucent substance of which the trunk is at this period 
composed. 
If now the eye is carried along the anterior surface of the 
embryo, there will be perceived, immediately below the 
anterior bulging indicative of the seat of the future cerebral 
hemispheres, a series of four laminae or leaflets, which, when 
