296 MENSCH. [Vol. XVI. 



which the new segments formed in the anterior part of the 

 chain are built up. 



The gradual transformation of the tissues of this segment 

 into embryonic tissue is well shown in a longitudinal median 

 section of this and succeeding segments of the chain. PI. XIII, 

 Fig. 17, represents a longitudinal sagittal section through seg- 

 ment 35, the embryonic region and the anterior part of stolon i, 

 Fig. 6. The plane in which the section represented in Fig. 1 5 

 was taken is shown at A, while the plane of Fig. 16 is shown 

 at B. The limit of this segment (35) is well marked anteriorly 

 by the dissepiment {dis.)\ posteriorly, however, the internal 

 limits of the segment are lost in the masses of embryonic 

 mesoderm {m.e.), which completely fill the body cavity at the 

 posterior extremity of this segment, and the external constric- 

 tions alone define its exact limit. The dorsal epidermis at B, 

 and posterior to this plane, shows quite a transformation when 

 compared with that anterior to the plane A, the cells appearing 

 considerably narrower and possessing more the characters of 

 embryonic ectoderm. Ventrally the character of this ectoder- 

 mal tissue, as seen in the longitudinal sections, is even more 

 distinct, and in the posterior part of this segment {e.e), in a 

 plane with the denser masses of embryonic mesoderm, the out- 

 lines of these ectodermal cells are quite apparent. The tissues 

 in the regions belonging properly to segment 35 are composed 

 of elongated spindle-shaped cells closely crowded together, but 

 having a clearly defined outline, particularly at the very poste- 

 rior border of this segment. More anteriorly their outlines 

 become gradually fainter, until, in a plane a little anterior to B, 

 they assume the appearance common to the region ventral to 

 the nerve cord in preceding segments. 



From a longitudinal section of segment 35 it is, therefore, quite 

 evident that the posterior part of this segment has become con- 

 verted into an embryonic center, in which new tissue is con- 

 stantly being added to the three embryonic layers, which new 

 tissue, in the process of formation, is carried back and forms 

 the most anterior embryonic segments of the chain. In this 

 way the formation of all the segments preceding stolon A, 

 PI, XIII, Fig. 6, can be accounted for. 



