16 



fcrr size ard are situated i^ a rinf^le erithelirl Ir.yer 

 r.rour.d the :-eriT:'hery of thr blartula (fi.rures 16 and IV 

 shcv; ?.pctic?s of blastulae five and eipht ard ere half 

 hours eld resreetively) , Thur , frc:- their resitirr, all 

 the cells v.'hich result fron the sep^entatic of the epp di- 

 rectly ray rrcrerly be refjarded as ferrying ectoderm; and 

 indeed "alreac y at this stage cf develonrent be desirnated 

 as such, v-ere it -rc-er to use the ter:- ectoderm before 

 the ar-nearance cf an inner gerr layer. The cells of the 

 blastosrhere are cclurnar in share a-'C at first all are cor- 

 T^aratively of the srre height; but finally those cells rt 

 the nosterirr e:-d become screv;hat taller than the rer-t. 

 This is the region v/here the endcder^i- vail be budded off. 



for;.: AT I ci: of the ekdcder;-. 



In Stomctoca the fopT^atio' of the endoderr tal:es nlace 

 by unipolar ingression, or ":he "hynctrone" method. The 

 latter ter~ was use: by i-.:etschnil:off in contradistinctien 



