THE NYMPHOID STAGE OF THE EMBRYO. 259 



unite behind the mouth and rapidly undergo a great reduction 

 in size ; they form the labium. 



There is no doubt in my mind with regard to the fate of the 

 procephalic lobes and fore-head, although the meagre facts 

 which have been hitherto recorded are by no means in perfect 

 accord with my view. Weismann is the only author who 

 claims to have observed the phenomena, and his observations 

 are somewhat vague, and in some points, I think, incorrect. 



The changes which take place during the involution of the 

 procephalic lobes will be more readily understood by a refer- 

 ence to Figure 44 than by the most lengthy description. They 

 are probably brought about by the rapid growth of the anterior 

 part of the stomodaeum ; and by the increased size of the 

 somites developed from the somatopleure, which causes the 

 prothoracic segment to advance towards the anterior extremity 

 of the embryo. 



I have sections which show that the procephalic lobes form 

 a double involution on each side, and, from the condition of the 

 great cephalic discs in the newly-hatched larva, I think it is 

 manifest that their antennal and optic rudiments are formed by 

 the invagination of the procephalic lobes. Although I have 

 not been able to demonstrate the invagination of the fore-head 

 at this period, it undoubtedly rapidly disappears, and the 

 invagination of this part is so clearly seen in the newly-hatched 

 larva, that I can see no reason to doubt that it forms the cavity 

 above the labrum already described (p. 41). This communicates 

 below with the sacs which enclose the antennal and optic rudi- 

 ments or discs. 



Weismann says: ' When the fore-head is definitely separated 

 from the procephalic lobes, it appears as an obliquely truncated 

 process, having the form of a four-sided prism. Its ventral 

 surface is convex, but neither this nor the upper flat surface 

 exhibits any median fissure. After the anterior maxillae become 

 parallel, the fore-head is bent towards the abdominal surface, 

 and rapidly assumes a position in which its anterior truncated 

 surface becomes ventral ; its ventral surface covers the mouth 

 and lies upon the primitive band, which is only separated from 



