84 WHEELER. [Vol. VIII. 



3-5 longitudinal rows in either lateral cord. In surface view 

 these rows may often be followed through one or two seg- 

 ments as continuous strings of cells. I assume that there 

 were originally four of these rows, but that owing to the 

 pressure exerted by the developing appendages on the lateral 

 edges of the cords and to a more rapid growth of the neuro- 

 blasts than of the germ-band, the primitive regular arrange- 

 ment has been considerably obscured. The neuroblasts are 

 polygonal in outline from mutual pressure. When they divide, 

 as they very soon do, their spindle axes are directed at right- 

 angles to the surface of the body. As soon as one cell has 

 been given off, the nucleus rests for a short time and then 

 again divides in the same direction. This process continuing, 

 a column of cells is budded off from each neuroblast and stands 

 at right angles to the surface of the germ-band. The divisions 

 do not take place simultaneously in all the cells although cor- 

 responding neuroblasts in either cord will frequently be found 

 in the same phase of caryokinesis, especially in the earlier 

 stages of their proliferation. A section (Fig. 27) through the 

 first maxillary segment of an embryo in Stage F shows that 

 each of the eight neuroblasts has produced a row of daughter- 

 cells. The large succulent mother-cells are evenly rounded on 

 their outer surfaces which are overlaid by the dermatoblasts. 

 Their inner faces are flat or concave and in every case closely 

 applied to the latest daughter-cell. The nuclei of the mother- 

 cells are spheroidal and take no deeper stain than the pale suc- 

 culent cytoplasm which surrounds them. Thq neuroblasts are 

 in all essential respects typical proliferating cells like the ter- 

 minal cells in plant-shoots and the teloblasts of annelids. The 

 daughter-cells (^') are at first characterized by their small 

 size, cuneate outline and deep stain. Their nuclei are con- 

 siderably flattened, probably from mutual pressure. These 

 characters are retained by the daughter-cells till they have been 

 pushed some distance from the neuroblast by later offspring, 

 when they become larger and considerably paler and assume 

 the appearance of definitive ganglion cells {g~). 



Turning now to a somewhat older embryo (Stage G, Fig. 28) 

 we see that the columns of daughter-cells have greatly increased 



