195 



and bony shields (e.g., armadillo) are developed from the 

 dermis. Skin-teeth or dermal denticles (of Elasmobranchs), 

 and ordinary teeth are formed mainly (bone basis, and core 

 of dentine or ivory) from the dermis, partly (enamel crown) 

 from the ectoderm. As indicated by these structures, 

 the skin is protective ; it is also highly sensory, it receives 

 impressions from the outside world, develops sense organs, 

 and from it are formed the main portions of the central 

 nervous system. Regulation of the body temperature, 

 excretion, and respiration (e.g., frog) are also functions of 

 the skin. 



Write a short general Account of the Muscles of the Body. 



There are two kinds, namely, myotome muscles, which 

 are developed from the inner walls of the myotomes, and 

 visceral (mesenchymatous) muscles, which arise from the 

 wall of the coelome (splanchnocoele). 



The myotome muscles, which have cross-striped fibres, 

 form the muscular layer of the body wall, and are concerned 

 with locomotion ; the layer being composed of zjg-zag 

 segments (separated by " connective tissue septa), this 

 permits of side to side wave-like bending. The development 

 of limbs in the higher Craniates results in varied movements 

 and corresponding modifications of the myotome muscles, 

 so that only traces of the original segmental arrangement 

 remain in the adult. The visceral muscles, which generally 

 have smooth or unstriped fibres, are those of the alimentary 

 canal, heart, blood vessels, and other parts. The heart 

 muscles are of the cross-striped type. 



How is the Body Cavity (Coelome) developed? Describe the 

 Body Cavity of an Adult Craniate. 



Solid mesoderm (in the trunk region on each side) is 

 derived and detached from the endoderm (wall of archen- 

 teron), and is segmented dorsally (protovertebrae or myo- 

 tomes) but not ventrally (lateral plate). The mesoderm 

 segments are therefore incomplete, and their continuous 

 ventral portions thus form a sheet on each side. (Cf. 

 Development of Chick, page 248.) Spreading downwards, 

 the two sheets meet below the gut and there coalesce as 

 one ; above the gut they become closely apposed (when 

 the dorsal myotome segments are separated off from them), 

 and their conjoint walls form the fold of mesentery to which 

 the gut (intestine) is attached. When the solid mesoderm 

 is later hollowed out by splitting, the cavity thus formed 



