1^6 WHITMAN. [Vol. I. 



often separate from one another for a short distance in front of 

 the teloblasts. Farther forward, each row becomes double, 

 then triple or quadruple, and at the same time its boundary lines 

 become less clearly defined, as shown in Figs. 8-ii. Fig. 7 is 

 a transverse section of the stage seen in Fig. 8, at a point where 

 the bands are still separated by a narrow interval. The rows of 

 cells are here simple, each showing a single cell in section. In 

 another section, taken just in front of this, where the bands have 

 already closed, three median cells (Fig. 6, nc) are seen in place 

 of the two shown in Fig. 7. It is possible that there has been a 

 duplication of cells in one of the median rows, but it is more 

 probable that the section has passed through two more or less 

 wedged-shaped cells belonging to the same row, but inversely 

 placed, as shown in the left neural row of Fig. 11. Fig. 11 rep- 

 resents a horizontal (frontal) section near the posterior end of an 

 embryo in which the germ-bands are fully closed. All the rows 

 are here simple, but towards the middle of the same embryo 

 (Fig. 10) we find the neural row doubled on one side and 

 tripled on the other. A little in front of the middle a still more 

 advanced condition is found (Fig. 9) ; for here not only have 

 the neural cells become smaller by division, but there is a plain 

 diff"erentiation into median (Long, commissures + median gan- 

 glia) and lateral (lateral ganglia) portions. 



A more complete and instructive picture may be obtained by 

 mounting the embryo entire, after stripping it from the yolk. 

 In such a preparation (Fig. 8), passing from the hind end for- 

 wards, we meet with successively higher stages in the develop- 

 ment of the nerve-chain, beginning with two simple rows of cells 

 {nc) and ending with well-defined ganglia. The neural rows 

 are so clearly delimited against the darker, nephridial rows, and 

 the steps in development follow in such a perfect ascending 

 series, that every doubt about the conversion of these rows into 

 the nerve-chain is removed. 



In Figs. 15—19, representing transverse sections from the 

 posterior end of an embryo just hatched, the neural rows have 

 united into a median flattened cord {11c) which shows a differ- 

 entiation into median {Ic) and lateral portions, as in Fig. 9. In 

 these sections, selected from a series beginning in the middle 

 of one somite and extending to the middle of the next in front, 

 the contrast in color between the neural and nephridial elements 



