NEUROBLASTS IN THE ARTHROPOD EMBRYO. 



WILLIAM M. WHEELER. 



As some months will probably elapse before I shall be able 

 to complete my study of Xiphidiian ensiferuin, I take advantage 

 of the opportunity kindly offered me by Professor Whitman to 

 publish a few observations bearing on the development of the 

 nervous system of Arthropods. 



In Xiphidium the nervous system begins to make its appear- 

 ance before the elongate blastopore closes and the fold of the 

 amnion and serosa envelops the head — at a time, therefore, 

 when the embryo is still in what I have called the "slipper" 

 stage. There may then be seen, scattered over the surface of 

 both procephalic lobes a number of pale circular spots, each 

 surrounded by a ring of the small cells forming the ventral 

 plate. The embryo still consists of a single layer of elements 

 except beneath the blastopore, where the meso-entoderm is 

 differentiating. In sections the pale circular spots are seen 

 to be due to centres of proliferation, each consisting of a few 

 enlarged cells. Surface study is rendered difficult as soon as 

 the envelopes have enclosed the head, and it is not till the 

 embryo has grown and stretched the overlying amnion that 

 the surface again becomes clearly visible. In the meantime 

 the nervous system must be studied in sections. 



Beginning with the ventral nerve-cord, I find that the 

 ganglia arise as paired thickenings of the ectoderm in the 

 manner so often described for Arthropods. The thickenings 

 are the lateral cords (" Seitenstrange ") — the region between 

 them, marked on the surface by a groove, is the median cord 

 (" Mittelstrang " of Hatschek). Carefully made transverse sec- 

 tions through either lateral cord are seen to consist in early 

 stages of two kinds of ectoderm elements : smaller cells with 

 rather deeply stainable elongate oval nuclei and four large suc- 

 culent cells with pale spherical nuclei. These four large cells, 

 the neuroblasts, lie side by side just beneath the smaller ecto- 



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