REPRODUCTION AND LIFE CYCLES 



419 



Thus far two germ layers 

 can be differentiated, an outer 

 layer of ectoderm and an inner 

 one of endoderm which lines the 

 archenteron, while the dimin- 

 ishing remains of the blasto- 

 coel lie between. This stage is 

 suggestive of those organisms, 

 like the coelenterates, which 

 characteristically possess only 

 two germ layers even in the 

 adult condition, and are there- 

 fore designated as diplohlastic. 



Mesoderm Formation 



The details of the further 

 development of the embryo 

 vary considerably, depending 

 upon the form studied, but 

 all of the higher forms above 

 the coelenterates produce a 

 third germ layer called the 

 mesodcrjyi. The elaboration of 

 mesodermal tissue may come 

 from either, or possibly both, 

 of the primary germ layers. 

 In all of the vertebrates, two 

 sheets of mesodermal cells are 

 formed, an inner splanchnic 

 layer associated with the inner 

 tube, or developing gut, and an 

 outer so7natic layer, which is 

 contiguous with the ectoderm. 

 Loosely scattered mesodermal 

 cells {mesenchyme cells), de- 

 rived from these more compact 

 layers, fill in the narrow spaces 

 between the gut and splanch- 

 nic layer and between the 

 somatic layer and ectoderm. 



mccUxllarx plate 



.Tn€.ciunary fold 



iwtochortt 

 mesoderm 



endoderm 

 enterpcoeli<t 



sctoderm 



YnedujcWaxy fokL 



iTiyotome/ 

 tnyoco€/l 

 denrjCLtome- 

 nephrotome 



notoebord; 

 e.ndoderni 



Sorrxxt'id. 

 mesoderm 



$plctnch.n\c 

 mesodema 



ectode-rm. 



>ieura.l cocnal 

 ocnd, tijcbe. 

 myotorne 

 rnyocoel 

 dermatome. 



fn<as«ntery 



Somatic and. 



splanchnic 



mesooLerm 



©ncCocC<sr-rn 

 .<SCtodernri 



Diagram of a generalized vertebrate to 

 show the origin and early differentiation of 

 the ectoderm, endoderm. and mesoderm. 

 (I) shows the mesoderm arising by means 

 of the enterocoelic pouches budfling off 

 from the archenteron. Above and between 

 these pouches lie the beginnings of the 

 notochord. In (II) the medullary plate has 

 formed the neural tube and the mesoderm 

 has become differentiated into regions 

 which will form somites (myotomes), kid- 

 neys, and linings of the body cavity. This 

 differentiation goes still further in (III). 

 (After McEwen.) 



