3S8 



WHEELER. 



[Vol. IV. 



derm elements in a plane parallel to the surface of the yolk and 

 the outer surface of the body. As every transverse section 

 through the ventral nerve-chain in this stage contains approxi- 

 mately four of these huge cells, I conclude that the embryo 

 presents eight longitudinal rows of neuroblasts. The rows 

 extend from the mouth to the anus, and are most clearly differ- 

 entiated anteriorly. When sections of the head are examined 

 it is found that the sporadic clusters of cells forming the pale 

 surface spots of the younger embryo have arranged themselves 

 as neuroblasts in eight rows in either procephalic lobe. Four 

 of these rows — those on either side of the stomodaeal orifice 

 and directly continuous with the rows of the ventral cord — 

 give rise to the brain proper, while four shorter lateral rows 

 form the optic ganglion. The retinal ganglion is delaminated 

 very early from the outer edge of the procephalic ectoderm. 

 From the first its elements do not resemble the neuroblasts 

 and appear to multiply irregularly. 



I give a slightly diagrammatic figure of a cross-section 

 through the posterior portion of a basal abdominal ganglion. 

 It is taken from an embryo somewhat older than the one just 

 described. The two lateral cords have been made to bulge 

 out beyond the general surface of the ectoderm by the pro- 

 liferation of the neuroblasts (5^-5*), each of which now sur- 

 mounts a pillar of smaller elements. These, the future ganglion 

 cells, are budded off from the inner ends of the neuroblasts 

 and become cuneiform from mutual pressure. There is a 

 marked contrast between the terminal and daughter cells, 

 not only in size and shape, but also in intensity of stain. 



