324 THE DEVELOPMENT OF THE NYMPH. 



and form part of the wall of a great pyriform sac extending 

 from the brain to the pharynx. 



In the pupa on the third day they have become sacs, the 

 corneal surface looks towards the interior of the sac, and is 

 concave, whilst the outer convex surface is covered by an ex- 

 pansion of the optic stalk which unites them with the brain 

 (see Fig. 46). 



Van Rees finds great fault with Viallanes' figure of the eye 

 disc, which represents it with the outer surface covered by a 

 provisional membrane, and states that the surface which looks 

 towards the exterior is covered by mesoblast. He has figured 

 and described the discs in their third stage as they appear in 

 the pupa, the concave corneal surface bounding the lumen of 

 a sac. During the evolution of the head, the eye discs again 

 become convex on their corneal surfaces. 



Van Rees gives a perfectly correct description of the con- 

 dition of the parts from which the head is developed, as they 

 are found in the last larval and early pupa stages, but, had he 

 examined them in a series of sections from larvae a few hours 

 younger, he would have found that Viallanes' figures are as 

 correct as his own. He says : ' The forehead and eye discs of 

 the adult larva extend from the brain, with which the latter 

 are connected by nerve stalks, to the pharynx. The eye discs 

 extend over the front of the brain and resemble mushrooms, 

 whilst the antennal discs, which are continuous with the edges 

 of the eye discs, at first form a tube and later a cone, with its 

 apex attached to the pharynx.' This description scarcely differs 

 from Weismann's. 



c. Changes in the Neuroblast, and Development of Peripheral 



Nerves. 



It is not my purpose at present to enter into the details of the 

 development of the nervous system, which will be more con- 

 veniently given in a future chapter, but to present to the reader 

 a review of the great changes which occur during the evolution 

 of the neuroblast. 



