No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 103 



seen in later stages, their morphological value as indicating the 

 presence of another segment in front of the protocerebrum is 

 somewhat doubtful. 



The three divisions of the protocerebrum may still be rec- 

 ognized in the transverse section (Fig. 40) of an embryo which 

 has passed beyond Stage J. In the median line at inc lies a 

 rounded and compact mass of cells which I regard as the prae- 

 oral representative of the median-cord cell-masses of the ventral 

 cord. A large cell, which I take to be a neuroblast, lies at the 

 outer periphery of this median cell-mass at a younger stage. 

 Structures at a corresponding position in the brains of other 

 insects {Hydrophilus, Mused) and likewise comparable to the 

 median-cord have been described by Heider ('89) and Graber 

 ('89, p. 49). It is not improbable that the brain neurilemmata 

 may have their origin near this median cerebral mass, just as 

 the neurilemmata of the ventral cord probably arise from the 

 non-ganglionic portions of the neural furrow. 



Two other interesting structures are shown in Fig. 40 : the 

 intraganglionic thickening {igl), now completely separated from 

 the integument and lying as a deeply stainable mass wedged in 

 between the optic ganglion and the second protocerebral lobe, 

 and a peculiar pale thickening at the edge of the optic plate {th). 



In later stages it is very difficult to locate the intraganglionic 

 mass, so that I am unable to decide whether it atrophies or 

 persists in a modified form as a portion of the brain. The re- 

 searches of Viallanes on Mantis would seem to lend great 

 probability to the former alternative. 



The thickening at the lateral edge of the optic plate is con- 

 stant in Stages G to J and somewhat later. It soon entirely 

 disappears, without taking any part in the formation of the eye, 

 so far as I have been able to observe. Does it represent an 

 abortive ocellus .-' 



The fully established optic nerve is shown in Fig. 41 {0. n). 

 In a much earlier stage it may be found as a delicate band of 

 cells connecting the posterior edge of the optic ganglion with 

 the optic plate (Fig. 38, 0. n). I agree with Viallanes, that it 

 seems to arise from the ganglion and to grow outwards into the 

 ommatidial layer ; for there is from the first a sharply defined 



