XXIV 



THE GASTRULA 



265 



not by a mouthless planula but by a two-layered embryo 

 with a mouth at one end, called & gastrula (Fig. 63). This 

 is a very important stage, since it exhibits in the simplest 

 possible way the essential characteristic of a diploblastic 

 animal a two-layered sac with mouth (Blp) and stomach 

 (U), the outer layer of cells (Ekt) being protective and 

 sensory, the inner (Ent) having a digestive function. The 



FIG. 63. A typical animal gastrula in vertical section, showing 

 ectoderm (Ekt), endoderm (Ent), enteron or digestive cavity (7), and 

 mouth (Blp). (From Wiedersheim. ) 



planula of a hydroid may be looked upon as a gastrula in 

 which the mouth has not yet appeared. 



Another very important difference is the fact that in uni- 

 cellular organisms reproduction is effected either asexually 

 by the fission of the entire individual, or, in the case of 

 sexual reproduction, by two entire individuals undergoing 

 conjugation. In multicellular forms, on the other hand, 

 single cells are set apart for sexual reproduction. 



