THE DEVELOPMENT OF THE NERVOUS SYSTEM. 479 



3. THE DEVELOPMENT OF THE NERVOUS SYSTEM. 



The origin of the nervous system from the primitive band 

 and procephalic lobes of the embryo has been sufficiently 

 established by numerous observers ; but the exact manner in 

 which the ganglia originate is still, I think, doubtful ; and the 

 embryo of the Blow-fly is not well adapted for the determination 

 of their origin, as the earlier changes take place with extreme 

 rapidity. 



The first appearance of the ganglia of the ventral cord, as 

 definite and recognisable structures, is identical with their first 

 appearance in Hydrophilus, as figured by Kowatevski [97] . 

 The} T are seen at first as definite groups of cells, occupying 

 only a very small portion of the primitive band. 



The generally received view is perhaps accurately expressed 

 by Balfour [43] ; he states that the ventral cord originates from 

 two longitudinal thickenings of the epiblast, one on each side 

 of the median line, which subsequently separate from the 

 integumentary epiblast, and give rise to the two lateral 

 strands of the ventral cord ; and that these subsequently 

 undergo segmentation and differentiation into the ganglia and 

 their connecting cords. 



So far as my observations go, the above statement is not 

 quite accurate. The primitive band is correctly described as 



Bibliography : 



187. HATSCHEK, B., 'Beitrage zur Entwickelungsgeschichte der Lepi- 

 dopteren.' Jenaisch. Zeitsch. f. Naturw., Bd. xi., 1877. 



188. REICHENBACH, H., 'Die Embryoanlage und erste Entwickelung des 



Flusskrebses.' Zeitsch. f. w. Zool., Bd. xxix., 1877. 



189. HEIDER, K., 'Ueber die Anlage der Keimblatter von Hydrophilus 

 piceus L.' Abhand. der Koniglichen Akad. der Wissenschaft zu 

 Berlin. Anhang zu den Abhand. (1885), 1886. 



Heider has since published (1891) a more extended work on the 

 development of Hydrophilus, which I only know from quotations. 



190. VIALLANES, H., 'Sur quelques points de 1'histoire du De"veloppe- 

 ment embryonnaire de la Mante religeuse (Mantis religiosa)! Ann. 

 Science Nat. Zool., seVie vii., torn, xi., 1891. 



191. WHEELER, W. M., 'A Contribution to Insect Embryology.' Journ. 



of Morphology, vol. viii., 1893. 



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