196 THE HEART. 



cavities) that give rise to the endocranium, the jaw and leg muscles, and the coxal 

 glands (head kidney). (Fig. 138, c.so.) 



There is a conspicuous territory, lateral to the cephalic lobes and thoracic 

 appendages, that presents little or no indication of segmentation. (Figs. 15, 16, 

 19, 21, 31-33, 141-156.) It is covered by a thin layer of ectoderm with numerous 

 underlying, oval cells, or fiber cells, that are eventually converted into muscles, 

 or peculiar bodies resembling blood corpuscles. See vascular area (Chapter XIII, 

 page 232.) 



Back of this region the lateral walls of the embryo are divided into distinct 

 segments. The peripheral margin of these segments, up to a comparatively late 

 embryonic period, ends in a kind of germ wall, where the advancing sheets of 

 ectoderm, mesoderm, and endoderm, or "yolk cells," merge into a common 

 primitive-streak-like thickening. (Fig. 134, g.w.) 



In the vagal and branchial metameres, the lateral plates of mesoderm con- 

 sist of definite somatic and splanchnic layers, which enclose separate coelomic 

 chambers. In these regions, the fiber cells are absent, but numerous blood cor 

 puscles are present, formed from liberated germ wall cells. 



As the lateral ends of the mesodermic segments approach the haemal sur- 

 face, they separate from the germ wall, take on a crescentic form and, uniting with 

 their mates of the opposite side, form the walls of a heart segment, or cardiomere. 

 The cardiac ostia represent the spaces between the anterior and posterior walls 

 of the adjacent segments. (Figs. 136, 137, h.) 



Only the lateral plates of the sixth thoracic, the chelarial, opercular, and 

 five branchial segments form definite, or permanent, cardiomeres. 



The coelomic cavities, at the haemal ends of the segments that form cardio- 

 meres, become partly shut off to form the pericardial chamber. The neural 

 portion of the ccelom, belonging to the five branchial segments, forms the five 

 great veno-pericardiac canals. 



As the margins of the lateral plates advance, the cardiac nerves follow after, 

 keeping close to the intersegmental thickenings of the ectoderm, thus reaching 

 the heart tube opposite the intersegmental ostia, a position which they retain 

 throughout life. (Figs. 115-151, s.c.n.) 



As the lateral plates of the cardiac segments advance over the yolk, they 

 expand, fan-like, into the unoccupied yolk surface in front and behind more 

 rapidly than along the true parallels of the yolk sphere. The result is that when 

 they unite on the haemal surface, the anterior heart segments lie farther forward 

 in the thoracic territory than their neural ends. This unequal displacement 

 gradually carries the anterior end of the heart tube forward till it almost meets 

 the anterior end of the forebrain, which is being crowded backward in the opposite 

 direction. (Figs. 17, 26, 31, 138, 157.) 



Thus in the later embryonic periods and in the adult, the original relation of 

 the cardiomeres to the neuromeres is greatly disguised, except in so far as it is 

 shown by the terminals of the segmental cardiac nerves. 



