THE FIBER CELLS. 233 



of tergo-plastrals arise from the endocranium, and five pairs of tergo-coxals 

 from the coxal joints of each thoracic appendage, two in front and three behind. 

 These muscles and the spaces between the developing gut lobes are important 

 conduction paths for the distribution of the fiber cells. 



The muscles arise at a very early period from the anterior and from the poster- 

 ior wall of each mesodermic segment, close to the somites. They form the only 

 suggestion of segmentation that is to be seen in the lateral plate area of the thorax. 

 (Figs. 142-151, hrn.rn 1 ' 5 .) 



At first the tergo-coxal muscles lie in a nearly horizontal plane, their median 

 or neural ends attached to the ectoderm between the bases of the thoracic ap- 

 pendages. (Fig. 142.) Their lateral ends gradually advance with the germ 

 wall toward the haemal surface, and as they swing into a nearly vertical position, 

 they cut the thoracic yolk mass into five great lobes, which ultimately become the 

 five main liver lobes, or enteric pouches, of the thorax. (Figs. 142-151, 

 hn.m I ~ s .) 



In stages A' and L, the fiber cells begin to scatter in different directions. 

 Many leave the surface and penetrate the interior of the thorax, following, in the 

 main, the channels between the yolk lobes formed by the hsemo-neural muscles. 



During stage L, embryos seen from the haemal surface show the fiber cells 

 as two large mottled patches on the anterior lateral surface of the thorax. (Fig. 

 149, a.v.) The clear oval area between them represents the cephalic navel, or the 

 depression where the remnant of the blastoderm is passing into the interior of the 

 yolk. 



The fiber cells congregate in great numbers along the sides of the cephalic 

 navel, and around the haemal ends of the first five pairs of haemo-neural muscles, 

 where they form dark colored, conical or wedge-shaped masses of cells. (Figs. 

 146-149, hn.m I ~ 5 .) 



It will be observed that up to the present time the fiber cells have a very 

 definite distribution, and except for a few scattering clusters they are absent 

 from the sixth thoracic, the vagus, and the abdominal segments. (Figs. 148 and 

 149.) 



The metamorphosis of some of these loose oval fiber cells into muscles is 

 very rapid and takes place a little after stage M, when the embryo is taking on the 

 trilobite form, and pigment has appeared in the eyes. At this period, the cells 

 stain more readily; the fiber is less distinct, forming finer parallel fibrils; the nucleus 

 takes up a central position; and the cells elongate somewhat and unite end to end 

 in irregular rows. (Fig. 132.) In the more advanced stages, a central canal 

 has formed, in which the dividing nuclei are arranged in a single row and the 

 beginning of cross striations is seen on the free, more or less pointed ends of the 

 cells. 



Two great masses of these muscle cells are formed on either side of the ceph- 

 alic navel and of the anterior end of the heart. From them are developed two 

 pairs of muscles. One pair, the inter-tergals, lies on either side of the heart. 



