8 2 WHEELER. [Vol. V 1 1 1. 



V. Neurogenesis in the Insecta. 

 I. TJie Nerve-cord. 



The first traces of the central nervous system of Xip}iidiiim 

 make their appearance at a very early stage, before the blasto- 

 pore is closed and while the envelopes are still incomplete. In 

 this stage (Fig. 2) surface preparations made according to the 

 methods given in the latter part of this paper, show a number 

 of pale spots scattered over the procephalic lobes. They fre- 

 quently occur also in the maxillary region, and, were it possible 

 to remove the amnio-serosal fold without injuring the surface of 

 the germ-band, would probably be found to extend still further 

 caudad. The meaning of these spots is apparent when sections 

 of embryos in Stages B-D are examined. In a transverse sec- 

 tion (Fig. 25) through the middle of the abdomen of an embryo 

 in Stage D, the ectoderm, which bulges out somewhat on either 

 side of the median line, is seen to consist of two kinds of ele- 

 ments. First, there are a few large, clear, polygonal cells with 

 spherical nuclei («<5.), lying in the deeper portion of the layer; 

 and second, a much greater number of small and more deeply 

 stainable cells {db}), differing in no essential respect from the 

 cells forming the remainder of the ectodermal layer, such as 

 the appendages. The latter cells have smaller, oval or cuneate 

 nuclei, which appear to contain more chromatin than the large 

 inner cells. While the small cells form a continuous layer, the 

 large elements make their appearance singly or in small clusters, 

 as seen in the figure. It is these pale clusters underlying the 

 darker cells which produce the pale spots seen from the surface. 



The large pale cells may be called neuroblasts — since it is 

 they that give rise to the purely nervous elements of the cord.^ 



1 The term "neuroblast" was originally used by Whitman ('78 and '87) to 

 designate the two offspring of the large posterior macromere of the Clepsine egg, 

 which give rise by a process of budding to two rows of cells — the " neural rows." 

 From these rows the nerve cord arises. His ('89) subsequently employed the 

 same term " neuroblast " to designate such of the offspring of the " Keimzellen " 

 as give rise directly by differentiation and not by further divisions to the ganglion- 

 cells, or, to use Waldeyer's term, to the neurons of the vertebrate central nervous 

 system. More properly the term would have been applied to the '• Keimzellen " 

 themselves, and by mistake it has been thus used by at least one recent writer 

 (C. L. Herrick, '92, p. 430, Fig. 10). Haeckel (Anthropogenie, 4th ed. p. 26S, 

 '91) uses 'neuroblast' in the sense of ectoderm in general. 



