67 



a ruirber of nuclei are scattered through the protrrlac::. The 

 •nuclei soon brcoine rove nimorous near the periphery; ard 

 ther cell v/alls bepin tc arrear ar rhovn ir Fif:ure 33. Tliese 

 cells are tc becor^e the ectodern, v.OiichjiE socr separated 

 fro^ the inner structureless rass by the development of the 

 iresof^lcea. Vo\; the ectodern^ forrs a distinct layer, ccr- 

 pcsed cf cclur^nar cells all of which are at first sir-ilar 

 i-^ structure and lie parallel to each other as shewn in 

 Figure 34. The differentiation of the ectodern cells takes 

 Place later. 



The forraticn of the ^er^inal layers i^ I^rj'ij-f^r^is 

 is different fro--- that which has penerally been described 

 for the developrent of Hydromedusaf , Ir the na.iority of 

 forr-s previously studied the differentiation took place either 

 by delarinat: on cr by cellular inpressicn, unipolar or rult--* 

 pclcr. These rethcds hove been well described and figured 

 by I/etsch^il'Off for a nuirber of species. 



Ir Aplaura ard rhcpalcnera there is found, accordinr to 

 letschril'off , a solid so-called rcrula staf^e destitute 



