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The infolded inner layer (lining the archenteron) is the 

 hypoblast or endoderm, and the outer layer is the epiblast 

 or ectoderm. 



When the blastula contains many yolk-laden cells, the 

 gastrula is mainly formed by an overgrowth (epiboly) of 

 animal-pole cells ; or it may be formed by a process of 

 delamination, by division of the blastula cells into two 

 layers. 



Archenterc 



Epiblas 

 Hypoblas 



FIG. 2. Section Of Gastrula. (Diagrammatic.) 



How is the Mesoblast or Mesoderm formed ? 



l?rom the archenteron, dorso-laterally, two folds arise 

 each of which contains a groove (primary body cavity). 

 These are the mesoblast pouches. They grow out and 

 become shut off from the archenteron. They meet and 

 unite and gradually extend so that ultimately the archen- 

 teron (now the enteron or food canal) is covered on the 

 outside (splanchnopleure) and the ectoderm on the inside 

 (somatopleure) by mesoderm ; an4 the extended cavity 

 between is the coelome or body cavity. That is one way ; 

 another is, from mesoderm cells separated off from the 

 endoderm which subsequently divide to form a structure 

 that is hollowed out ; the hollow being the ccelome or body 

 cavity. 



Which Structures or Organs are formed from (1) Epiblast 

 (Ectoderm), (2) Hypoblast (Endoderm), and (3) Meso- 

 blast (Mesoderm)? 



1. The Epiblast forms the epidermis, nervous system, 

 sense organs (partly), infoldings at either end of the gut 

 (fore-gut or stomodaeum and hind-gut or proct*bdaeum). 



2. The Endoderm forms the mid-gut (mesenteron), the 



