222 PROF. HUXLEY ON THE AGAMIC REPRODUCTION 



cephalic portion of the blastoderm beginniiig 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 a median fissure into two lobes, which play an important part 

 in the development of the head, and will be termed the " procephahc lobes." I have 

 abeady* made use of this term for the corresponding parts in the embryos of Crustacea. 



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 mai'gin. 



It is in embryos between this and xwth 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 Tooth of an inch in length (PI. XXXVIII. fig. 1, la), the proce- 

 phalic 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 

 oesophagus can be traced ascending and passing backwards with a gradual curve. 



Behind the cephalic region, the thoracic blastoderm passes nearly horizontally back- 

 wards, and already presents traces of a division into its three somites. Its upper surface 

 is close to the pseudovitelline membrane, and consequently is covered by but a very thin 

 layer of yeUc-like granules. 



At the end of the rudimentary thorax the blastoderm is suddenly folded forwards, so 

 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 dii'ection, for a short distance, — the extreme end 

 being fijiaUy 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 t) are more slender and elongated than the others, and are bent inwards near 



* 'Lectures on General Natural History,' Med. Times and Gazette, 1856-7. 



