ZOOLOGY 



SECT. 



>4S 



ils 



of ventral plates, right and left, unite to 

 form the single unsegmented coelome of 

 the adult, their walls giving rise to the 

 coelomic epithelium. 



At the same time the cells of the 

 splanchnic layer of the protovertebrae 

 become converted into muscular fibres, 

 which nearly fill the myoccele, and give 

 rise to the myomeres : the myocommas 

 arise from the adjacent anterior and 

 posterior walls of the protovertebrae. An 

 outpushing of the splanchnic layer, at 

 about the level of the ventral surface 

 of the notochord, grows upwards be- 

 tween the myomere externally and the 

 notochord and nerve-tube internally : 

 from the cells lining this pouch the con- 

 nective-tissue sheath of the notochord 

 and neural canal arises, and perhaps 

 also the fin-rays. From the parietal 

 layer of the protovertebrae is formed the 

 derm or connective-tissue layer of the 

 skin. 



The larva increases in size, and be- 

 comes very long and narrow, with a 

 pointed anterior end and a provisional 

 caudal fin posteriorly (Fig. 763, c). As 

 growth proceeds, new segments are added 

 behind those already formed, the noto- 

 chord grows forwards to the anterior end 

 of the snout, and the eye-spot (au.) and 

 olfactory pit appear, the latter as an 

 ectodermal pit which communicates 

 with the neurocrele by the still open 

 neuropore (np.). The mouth (m.) attains 

 a relatively immense size, still remaining 

 on the left side. 



Additional gill-slits arise behind the 

 one already mentioned : they all make 

 their appearance near the middle ventral 

 line, and gradually shift over to the 

 right side : at first they correspond 

 with the myomeres, so that' the seg- 

 mentation of the pharynx is part of the 

 general metamerism of the body. Alto- 

 gether fourteen clefts are produced in a 

 single longitudinal series. Above, i.e., 



