Nerve Rudiments 



95 



along the primitive streak, and the two main nerve- 

 cords arise in the thickened ectoderm on either side 

 of this groove (fig. 6 1 N). A pair of swellings in 

 each segment develop into the nerve-centres of the 



Fig. 6i.— Cross section of embryo of German Cockroach. S. serosa; A. 

 amnion ; E. ectoderm ; M. rudiment of nerve-cord ; Af. mesodermal pouches. 

 Magnified 500 times. From Nussbaum in Miali & Denny. 



ventral chain, those of the head lobes forming the 

 rudiments of the brain. The special sense-organs 

 are all due to the modification of ectoderm cells. 

 The simple eye of an insect arises as a cup-shaped 

 inpushing of the ectoderm ; the invaginated cells 

 lengthen and become changed into the retinal elements 



