No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. lOI 



in their slight affinity for stains; they differ in the more 

 elongate shape of their nuclei and cytoplasm. The dark layer 

 enclosing the optic ganglion on all except its innermost face is 

 the optic plate and will give rise to the compound eye. Passing 

 towards the median line in these sections (especially in Fig. 33) 

 two other thickenings may be distinguished {pc-, pcZ) — the 

 second and third protocerebral lobes. 



The cross-section, Fig. 36, runs through the labrum of an 

 older embryo (Stage F) and shows a considerable advance in 

 the structure of the brain. The three protocerebral lobes are 

 distinctly marked out. In the second and third, the neuro- 

 blasts have arranged themselves in a row and have budded 

 forth strings of ganglion cells. In the first lobe [pc'^ [P-g-'^ no 

 teloblastic arrangement is ever present. The cells are small 

 and narrow and early assume a radial arrangement around the 

 Punktsubstanz core at the base of the mass. The cells of the 

 optic plate, which stand away from the surface of the ganglion, 

 already show a tendency to differentiate in that they have 

 become smaller and narrower than the dermatoblasts covering 

 the two other lobes of the protocerebrum. At the juncture of 

 the second with the third lobe, several large dermatoblastic 

 cells are intercalated {igl). They are continuous with the 

 dermatoblasts covering the second protocerebral lobe. This 

 intercalated mass is called by Viallanes the boiirrelet ectoderni- 

 ique inti-aganglionnaire . I shall call it the intraganglionic 

 thickening. 



A still more advanced stage in the development of the brain 

 is seen in Fig. 37 (Stage G). This section passes above the 

 base of the labrum. Owing to the active proliferation of the 

 neuroblasts, the mass of the protocerebrum is greatly aug- 

 mented. The Punktsubstanz has made its appearance as a 

 confluent mass.^ The optic plate is much thickened and its 

 small cells are about to arrange themselves to form the 

 ommatidia. The intraganglionic thickening (igl^i presents 

 an interesting appearance. The edge of the optic plate is 

 united with the inner edge of the optic ganglia, but between it 



1 1 have seen nothing to corroborate Cholodkowsky's view ('91) that there are 

 at first three distinct and separate pairs of Punktsubstanz masses in the brain. 



