22 



DIFFEREMTIATIC;- OF THE ECTCDERMAL CELLS. 



I'her the Irrvr. is ab'^iit tv/enty-f^our hourr old r.rd 

 about the sr.'^e tl-e that the PTiclrderrnl tissue hoFins tr 

 arrar.rp itself into the definite irir'er rerir layer, a d:f- 

 fere'^tiaticr corr^PTice?, ir the ectcderral tissue. The inter- 

 stitial ce]ls row ra!-e their arrearance here a-d there by 

 crcv'dirp ir betveer the base? of the ectcderral ce!ls. 

 These latter cells v;hich heretofore r/ere straij^ht cylin- 

 drical structures v;ith their sides parallel to each other, 

 now becc^e ^-cre irre.pular; sone aspu'^e '^cnical f crTrs , others 



CO 



s'^i^dle shaiies avordinp to the rressure of the neiphbor- 

 inp cel?.s. Also, about this ti-e, or a little later, STiall 

 oval refractive ^odles ''al'e their a'^-^eara-ce urually ir 

 the i'^terstitial cells, occasio-ally i?" the ectoderra" cells 

 also. These srall cvcid structures rrarually ^-ush their 

 ccy tov;ard the exterior, ard finally cop to be situated 

 i-. or betv.-eer the ectoderr?! cells rt the surface. They 

 are develored irt^ -pratocysts . 



