232 EARLY STAGES OF ARTHROPOD AND VERTEBRATE EMBRYOS. 



few show a larger number, as in Fig. 159. In this specimen, the anterior 

 plates, which apparently belong to the post-branchial metameres, are directed 

 downward and forward as if they were growing around the egg below the gill 

 plates, just as the post-vagal segments of the arachnids encircle the egg on the 

 haemal side of the appendicular arches. (Figs. 17, 19.) The segments decrease 

 in length caudad, and the most posterior ones are directed downward and back- 

 ward toward the closing telopore, forming a distinct welt on either side. 



The lateral plates quickly disappear, so that it was not possible to follow them 

 into later stages. 



Although these results are very meager, it nevertheless seems probable that 

 the figures in question are to be regarded as a faint recurrence of the concrescing 

 segmented lateral plates of mesoderm so conspicuous in the arachnids. 



The Fiber Cells. One of the chief products of the germ wall is a thick band 

 of rounded or oval cells, that we shall call fiber cells. They lie in the first five 

 thoracic segments in an intermediate zone median to the germ wall. They have 

 a remarkable structure and history; some give rise to definite muscles; some 

 persist in the adult as a peculiar type of spindle shaped semi amoeboid cells re- 

 sembling blood corpuscless; others, after forming muscles, degenerate during the 

 later embryonic and larval stages. 



In the earlier stages, the fiber cells cannot be distinguished from the other 

 cells in the germ wall. (Figs. 128 and 129.) In stages J and K, they form a 

 broad, thick band of large oval cells, rather loosely arranged, and presenting a 

 very striking appearance. (Fig. 131, A.f.c.} Each cell contains a small eccentric 

 nucleus and a highly refractive, colorless fiber. The latter may run in a regular 

 spiral direction, filling the entire cell, or it may form regular loops arranged in 

 compact bundles that stand at various angles with each other; a, />, h and c. 

 Aside from the fiber, the cells appear empty and colorless, although in some 

 cases they may have a dense, slightly colored envelop, / and d. 



In stage G, the fiber cells are clearly visible in surface views as a dark inner 

 border to the germ wall, and extending from the cheliceral segment, where it 

 is especially enlarged, to the anterior border of the sixth segment. (Figs. 141- 

 144, a.v.) 



The band increases in width and continues to advance toward the haemal 

 surface. In stage H, it forms an equatorial girdle, the two extremes having 

 meantime united behind, and almost united in front. (Figs. 141 and 144, a.v.) 

 After it passes the equator the posterior limb moves rapidly forward, swinging 

 into a haemo-neural direction, thus shifting the center of the closing ring toward 

 the anterior haemal portion of the thorax. (Figs. 147 to 149.) 



Meantime the yolk mass of the thorax, and a little later that of the abdomen, 

 divides into distinct lobes, the future enteric pouches. Important agents in bring- 

 ing this about are the haemo-neural muscles of the thorax. There are eleven pairs 

 of these muscles attached to the middle of the dorsal shield in the adult, making 

 five pairs of complicated markings on its inner surface. (Fig. 155.) Six pairs 



