298 MENSCH. [Vol. XVI. 



similar to those comprising the remainder of this tissue. The 

 appearance of the ventral cord in this section is not unlike the 

 appearance of the cord in sections of the parent stock passing 

 through the region of the cord between the ganglia, except that 

 in the parent stock these strands are never so widely separated. 

 This section is, however, taken through a plane, in which, in a 

 more mature segment, a ganglion is present, and the examina- 

 tion of successive sections of this segment will demonstrate the 

 entire absence of a ganglion in a segment of this stage of 

 development ; neither are the nuclei surrounding the cord more 

 densely crowded than they are in any other parts of this tissue. 

 The entodermal cells show no important differences from those 

 of the preceding section, which represents a zone equally active 

 in growth. 



PI. XIV, Fig. 20, represents a section through the segment 

 immediately following the one just described, or the second 

 segment of the chain. The most important characteristics of 

 a section of this segment, as compared with the preceding, 

 appears in the greater thickening of the ectodermal tissue and 

 the scanty appearance of the mesoderm. Neither of these 

 layers presents structures that would indicate any difference 

 in the age of this and the preceding segments, the tissues of 

 both being quite alike in general arrangement and appearance. 

 The ectoderm of this segment has increased considerably in 

 thickness and would indicate, by the more crowded appearance 

 of the nuclei, a region in which the cells are smaller, and hence 

 a segment in which growth seems more active than in the pre- 

 ceding segment. Besides this, however, no differences can be 

 distinguished in a transverse section. The narrowing of the 

 band of mesoderm is confined mostly to a lateral decrease in 

 the tissue, while dorsally and ventrally no difference can be 

 observed from that of the preceding segment. 



The structural differences of these two segments are made 

 more apparent in the longitudinal sections of PI. XIII, Fig. 17. 

 The plane of PI. XIII, Fig. 18, is represented at C; that of 

 PI. XIV, Fig. 20, at D. If these two segments be compared, 

 it will be seen that in passing from plane C to plane D the 

 dorsal ectoderm becomes rapidly thicker, and the cells appear 



