170 Embryonic Development of Chironomiis 



Nerve- 

 cord. 



way by an infolding of ectoderm from the future moutli 

 (fig. 126). 



AVliile tlie fore- and hind-gut are forming, the embryo 

 has begun to shorten, and in a few hours tlie tail-end 

 retreats to the hinder pole of the egg, while the body 

 becomes almost straight (fig. 125). 



The details of the formation of the nerve-cord cannot 

 be followed with advantage in Chironomus-eggs, which 

 are small and hard to orientate. The main features 

 of the development, so far as we have been able to 

 observe them, agree with the beautiful results obtained 

 by AVheeler in Xiphidium (1893). 



Large cells appear on the 

 deep face of the ectoderm 

 of the ventral plate. From 

 these are derived by pro- 

 liferation ganglion - cells 

 which arrange themselves 

 as columns of daughter-cells. 

 Two lateral masses are thus 

 formed, and we have seen 

 indications of a middle 

 element. The masses of nerve-cells grow rapidly, and 

 are mainly responsible for the prominent lateral tracts 

 already mentioned. There are at first as many ganglia as 

 segments ; they are large, extend throughout the seg- 

 ments, and are only interrupted by the intersegmental 

 constrictions. Connectives and commissures form later. 

 The neurilemma is an epithelium derived from the ecto- 

 derm. The original fifteen ganglia behind the brain are 

 gradually reduced to twelve, the suboesophageal ganglion 

 of the larva being a comj)lex of three, and the last abdo- 

 minal a complex of two. During development the anterior 

 ganglia are always in advance of those further back. 

 The development of the brain is more complicated and 



Fig. 124.— Transverse section of 

 embryo, showing sexual germs (sjr), 

 IDroctodaei^m or hind-gut (pr). 



