222 PROF. HUXLEY ON THE AGAMIC REPRODUCTION 
cephalic portion of the blastoderm beginning to extend upwards again over the anterior 
face of the germ, so as to constitute its anterior and a small part of its superior wall. 
This portion is divided by å median fissure into two lobes, which play an importa part 
in the development of the head, and will be termed the a procephalic lobes.” I have 
already* made use of this term for the corresponding parts in the embryos of Orustacea. 
. The rudimentary thorax presents traces of a division into three segments; and the 
dorso-lateral margins of the cephalic blastoderm, behind the procephalic lobes, have a 
sinuous margin. 
It is in embryos between this and råsth of an inch in length that the rudiments of 
the appendages make their appearance; and by the growth of the cephalic, thoracic, and 
abdominal blastoderm, curious changes are effected in the relative position of these 
regions. 
In embryos about z$5th of an inch in length (Pl. XX XVIII. fig. 1, la), the proce 
phalie lobes are so completely bent backwards as to lie close against the tergal surface 
of the rest of the cephalic blastoderm, so that no pseudovitelline granules can any more 
be seen in this region of the body. At the same time the lobes have enlarged, and extend 
back as far as the base of the fourth pair of visible cephalic appendages. Their infero- 
lateral angles are rounded and produced, forming an elevation which appears to be the 
rudiment of the eye. 
- Below the anterior extremity of the embryo, the blastoderm is produced on the median 
line into a tongue-like process (Jb), whose inferior part eventually becomes the labrum, 
while superiorly it sends a triangular process (the rudiment of the clypeus) into the 
interval between the procephalic lobes. 
Immediately behind the labrum, the blastoderm curves at first downwards, and then 
sharply upwards and backwards, to a little beyond the line of the posterior edge of the 
procephalic lobes. 
The whole of this portion of the blastoderm belongs to the head. In the re-entering 
angle between it and the labrum the mouth is placed; it is a small aperture, whence the 
cesophagus can be traced ascending and passing backwards with a gradual curve. 
Behind the cephalie region, the thoracie blastoderm passes nearly horizontally back- 
wards, and already presents traces of a division into its three somites. Its upper surface 
is elose to the pseudovitelline membrane, and consequently is covered by but a very thin 
layer of yelk-like granules. 
At the end of the rudimentary thorax the blastoderm is suddenly folded forwards, 8° 
that the sternal surface of the hinder part of the future abdomen is almost in contact 
with that of the thorax. Having come opposite the anterior edge of the thorax, it is bent 
backwards, at right angles to its previous direction, for a short distance,—the extreme end 
being finally folded parallel with this part, and with its apex towards the head. 
- The great mass of the yelk lies over the abdominal blastoderm, in the space left between 
it and the pseudovitelline membrane. The appendages present a singular and beautiful 
uniformity. No trace of the pigment of the eyes is to be seen. The next anterior pair of 
appendages (a £) are more slender and elongated than the others, and are bent inwards near 
* * Lectures on General Natural History,’ Med. Times and Gazette, 1856-7. 
