358 



F. H. EDGEWORTH 



of the primordium consists solely of yolk-laden cells, the 

 posterior part of muscle-cells. 



In a 10-5 mm. embryo (Text-figs. 6-9) the pericardium has 



Text-figs. 1-5. 

 Embryo 9-5 mm., transverse sections; Text-fig. 1 is the most anterior. 



l' br. later at pi. 



c^^-^--^f^f ^^>M 



Z gilt de/h 



lyOLcim pi 



hypobr. 



Abbreviations to Text-figures. 



and. cap., auditory capsule, ceratohr., ceratobranchial. coraco-hy., 

 Coraco-hyoideus. coiist. br.. Constrictor branchialis. epi-hr., epi- 

 branchial. genlo-cor., Genio-coracoideus. g. j)etros. ix, ganglion 

 petrosuni ix. hypobr. ftp. ms., primordium of hypobranchial 

 spinal muscles, lateral pi., lateral plate, lev.. Levator arcus 

 branchialis. in. pL, muscle-plate. Mx., myotome x. Nr., 

 nerve x. operc, opercular fold, parachord. r., parachordal 

 cartilage, pericard., pericardium, peric. perit. duct, pericardio- 

 peritoneal duct. /////'•• thyroid body. 1st vert, arch, 1st vertebral 

 arch. Roman numerals, cranial nerves. 



retreated a little, and its anterior end is 56 /u behind the thyroid. 

 The primordium of the hypobranchial muscles has separated 

 into anterior and posterior parts — the (lenio-coracoideus and 



