No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. IO9 



ultimately supplant certain larval sense-organs. So far, how- 

 ever, as the Hexapoda are concerned, Patten's statement is, to 

 say the least, inapposite, since, as I have pointed out, both 

 brain and nerve-cord arise from peculiar ectodermal cells — 

 the neuroblasts — which under no circumstances can be re- 

 garded as primitively sensory. They are simply generalized 

 cells, like the teloblasts of worms and the meristem of plants. 



The development of the nerve-cord in the Hirudinea and 

 Oligochseta agrees more closely with the conditions seen in in- 

 sects. As Whitman has shown for Clepsine (;87), and E. B. Wil- 

 son for Lwtibriais ('89), the nerve-cord is proliferated forward 

 from a pair of neuroteloblasts situated at the posterior end of 

 the germ-band. Hence, in these worms, the whole of the nerve- 

 cord is condensed, as it were, into two huge mother-cells, 

 whereas in the Insecta it is condensed into a single layer of 

 hucre cells. There are reasons, however, for believing that 

 this layer is, in part at least, derived from a few large 

 cells situated just in front of the anus, and therefore corre- 

 sponding to the Annelid neuroteloblasts.^ That there are only 

 two rowt of these cells in Annelids, while there are eight in 

 insects, can form no very serious objection to their homology, 

 as I pointed out in my preliminary note ('90°). 



Certain conditions in the Crustacea also lend probability to 

 the view that the Hexapod neuroblasts may be budded forth 

 from a pr^-anal row of teloblasts.^ Patten ('90) has pointed out 

 in Cyniothoa a row of proliferating cells which form ectoderm, 

 and Nusbaum ('9l) has described a very similar condition m 

 Ligia. I have observed the same phenomenon in Porcellio, 

 and believe it to be of general occurrence throughout the 

 Isopoda. The cells are budded forth so as to form regular 

 transverse and longitudinal rows. Reichenbach describes and 



1 What I have called the neuroblasts in insects therefore correspond to the 

 "neural cell-rows " in Annelids (the cells up. c. in E. B. Wilson's Fig. 59, PI. XIX, 

 and the cells nc. in Whitman's Figs. 9 and 11, PI. V). 



2 The neuroblasts of the last row (tb., Fig. 56, PL VI) in the nerve-cord of 

 XipJndnnn are always distinctly larger and clearer than the neuroblasts of the 

 Remainder of the cord. They may be true neuroteloblasts and gwe nse to the 

 Teurobiasts but as I have never found unmistakable caryokmetic figures m 

 Lm am still in doubt as to their homology with the neuroteloblasts of worms. 



