THE DEVELOPMENT OF THE NYMPH. 327 



The corpora fungiformia and antennal ganglia are differen- 

 tiated from the inner or smaller portion of the hemisphere by the 

 third day of the pupa, but the ventral cord between the infra- 

 cesophageal and thoracic ganglia is not developed until the 

 separation of the head and thorax is well advanced. Previously 

 to the forward movement of the head these remain closely 

 united with each other as in the larva. 



Weismann [2] observed the pyriform condition of the hemi- 

 spheres in the pupa, and described the manner in which they 

 are divided mto two parts by a groove ; and Viallanes [27] 

 has given many particulars on the changes which the optic 

 ganglia undergo, and some of his observations and figures lead 

 me to believe that, if he had not been misled by received views, 

 which, as I have shown elsewhere, have no substantial basis, 

 he would probably have discovered the manner in which the 

 retina is developed in Insects. 



Of the optic stalk (Stiel) which unites the optic disc to the 

 hemisphere, Weismann says : ' It still appears on the fifth day of 

 the pupa as a nervous cord, but on the twelfth day it can be no 

 longer seen,"* He concludes, however, that it has by this time 

 spread out into an invisible layer over the whole surface of the 

 ganglion. That he should have arrived at such a conclusion is 

 scarcely consonant with the general careful character of his 

 work. If, as he states, and as is certainly the case, the 

 optic stalk disappears entirely between the fifth and twelfth 

 day, the opinion that the radial striae (which, he remarks, ap- 

 pear later between the optic disc and the optic ganglion) are 

 the nerve fibres which existed in the eye stalk is not based 

 upon any evidence, and appears to me contrary to the observed 

 facts. My observations lead me to the conclusion that the 

 whole of the nerve fibres ot the optic stalk disappear, and 

 that nothing remains but a thin layer of connective tissue, 

 uniting the capsule of the optic ganglion with the edges of the 

 optic disc, within which the retina grows outwards until its 

 outer surface comes into contact with the inner surface of the 

 parts developed from the optic 'disc— my dioptron. 



* These times refer to Sarcophaga, which is a week longer in the pupa 



than the Blow-fly. 



22 



