THE DEVELOPMENT OF THE NERVOUS SVSTEM. 479 
8. 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 Kowalevski [97]. 
They 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 [48]; 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. HatscHrK, 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. HEIprER, K., ‘Ueber die Anlage der Keimblatter von Hydrophilus 
piceus L.’ Abhand. der Kéniglichen 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 histoire du Développe- 
ment embryonnaire de la Mante religeuse (AZamtis religiosa). Ann. 
Science Nat. Zool., série vii., tom. xi., 1891. 
191. WHEELER, W. M., ‘A Contribution to Insect Embryology.’ Journ. 
of Morphology, vol. viii., 1893. 
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