104 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



similar origin. This is well demonstrated in the development of the 

 brain of Scolopendra, 



Among the higher arthropods, as Snodgrass (1938) has emphasized, 

 the more primitive stages in the brain development are generally not 

 shown in embryonic recapitulation, for the protodeutocerebral centers 

 are usually proliferated from the ectoderm as a unified ganglionic cell 

 mass, as in the Onychophora and in many of the Annelida. It is observed 



Fig. 47. — Orthopteran. Cross section of head through proto cerebrum, (am) Amnion. 

 (ant) Primitive antennal coelom. (ect) Ectoderm, (eye) Eye plate, igglc) Ganglion 

 cells, (lob) First, second, and third brain lobes. {Ir) Labrum. (wes) Mesoderm, (neur) 

 Neuroblast, (opg) Optic ganglion, {stom) Stomodaeura. 



by Baden (1936) and Roonwal (1937), however, that the brain of the 

 grasshopper {Melanoplus, Locusta) is formed from five pairs of ganglionic 

 centers, three of which give rise to the protocerebrum and the optic lobes, 

 and the other two to the deutocerebrum and tritocerebrum, respectively. 

 The brain develops in the cephalic lobes at about the same time that the 

 ventral chain is formed. A thickening of the ectoderm is followed by the 

 development of neuroblasts, thus establishing the neurogene and dermato- 

 gene layers. Three pairs of ectodermal thickenings can be distinguished, 

 corresponding to the protocerebrum, deutocerebrum, and tritocerebrum. 

 These are all connected with each other. The protocerebrum, by far 

 the largest, is nearly as wide as the head lobes. Laterad and contiguous 

 to it is an epidermal layer. Likewise on the median line undifferentiated 

 ectodermal tissue is present (Fig. 48C). The deutocerebrum is repre- 

 sented by two thickenings, one on each side of the stomodaeum. Behind 

 the mouth opening is the tritocerebrum in the form of two swellings 

 separated by still undifferentiated ectoderm. As in the ventral nerve 

 cord, the neuroblasts give rise to columns of daughter cells which develop 

 into ganghon cells. The protocerebrum now becomes transversely 



