VERTEBRATA ACRANIA. 



149 



from the archenteron. The first of these pouches is unpaired 

 and median, the remainder are paired and lateral. They develop 

 in order from before backwards. All these outgrowths become 

 separated from the gut (which may now be called the mesenteron), 

 and are known as meso- 

 dermic (mesoblastic) 

 somites, since they in- 

 dicate the segmenta- 

 tion of the body. 

 Other such somites 

 are added later as en- 

 dodermic outgrowths, 

 but their cavities 

 never communicate 

 with that of the gut. 

 The mesodermic telo- O 

 blasts give rise to part 

 of the mesoderm of 

 the tail-region. The 

 somites soon become 

 divided into dorsal 

 sections, the cavities 

 of which collectively 

 constitute themyocwle, 

 and ventral sections, CO 

 the cavities of which 

 collectively constitute 

 the splanchnocceh. The 

 outer wall of each sec- 

 tion consists of somatic 

 mesoderm, and its 

 inner wall of splanch- 

 nic mesoderm. The 

 splanchnic walls of 

 the dorsal sections 

 are converted into ^ 

 the lateral muscles, 

 and their somatic 



walls into the dermis. The sheath of the notochord, neural sheath, 

 .and septa are formed from outgrowths of these dorsal sections. 

 The myocoele is mostly obliterated in the adult, exwpt in the 



CNIVEBSITT 



