No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. ^t^ 



The remaining cells which cover the neuroblasts and extend 

 down between them in the median line, give rise to purely 

 integumental structures and may therefore be called dermato- 

 blasts. The two thickenings of the ectoderm are to become 

 the lateral cords (Seitenstrange). They extend from the 

 anterior edge of the eleventh abdominal segment, just in front 

 of the anus, to the mouth, where they diverge and pass without 

 interruption into the brain. The groove which separates the 

 lateral cords and which is very faint in Fig. 25, is the neural 

 furrow (Primitivrinne). It appears soon after the closure of 

 the blastopore and takes the place of this depression. It is 

 deepest anteriorly. 



All the neural structures develop in an anteroposterior direc- 

 tion, beginning with the brain ; hence different stages in the 

 development of the lateral cords may be studied in the same 

 embryo. Fig. 26 shows a section passing through the first 

 abdominal segment of the embryo from which the section 

 in Fig. 25 was taken. Here we see a distinct advance in 

 structure. The neural furrow {n.g) is more clearly marked 

 and the neuroblasts (71b), four in either lateral cord, have 

 arranged themselves side by side in a regular layer in the 

 deepest portion of the ectoderm. Over them the dermatoblasts 

 {db) also form a single regular layer, while the cells lying in 

 the median line on either side of the neural furrow have grown 

 more elongate. Sections further forward show essentially the 

 same conditions — the neuroblasts which were at first differ- 

 entiated as small clusters or as isolated cells, have arranged 

 themselves throughout the anterior portion of the embryo as 

 an even layer entad to the dermatoblasts. 



The further changes in the development of the nerve-cord, 

 are brought about — first, by a proliferation of the neuroblasts ; 

 second, by a proliferation of the dermatoblasts and a deepening 

 of the neural furrow ; third, by the development of the median 

 cord; fourth, by the formation of the connectives and com- 

 missures, and fifth, by the development of the neurilemmata. 

 These changes which occur simultaneously may be described 

 singly for the sake of convenience. 



As cross-sections show, the neuroblasts are arranged in from 



