82 Cell Interactions during Growth and Morphogenesis 



sponse to an inductive stimulus from the male. If too long a time elapses 

 before rece.vmg the signal, the female ceases to be responsive and w 1 

 no longer produce oospheres. 



stage. For example, only after the gnarled, branched shoots of the male 

 Achlya have grown out toward the female sac and plastered themselves 

 around rt can the male produce the hormone which induces oosphere for- 



THE EMBRYONIC ORSANIZER 



The most complex inductive mechanisms are found in vertebrate 

 embryos; one example is the embryonic organizer demonstrated by Dr H 

 te^eZ'^nr m' '°","'r u' ™°''^™ embryology. Spemann was »: 



or Trlduce ^,1 T Z °^ "^'"'^^ '^'^"^"'^ °^ ^" '"'^'y ™"'d contain 

 or produce all the cell types necessary for the construction of a complete 

 normal adult (see Chapter 6). He cut salamander embryos in hfl fai 

 var^us stages of development and found that, before gast'rulalL both 

 ab r ntW T rr '•'""' ""l' '*"" "^'"^ S^^*™'^"°" both developed 

 was to 'sne rV* ^^''^ *^ ^''P"^"^ f°^ "°™«' development 



tTon ind /I '""^''°' " '"'" '' ^"'°"^ '™« during gastrula- 



t on and, to h,s surpnse, one of the halves invariably gave rise to a com- 



norL T ^T : exammation. he found that the half yielding a 



normal product was always the one that contained the dorsal lip of The 

 bias opore> and that the half that degenerated always lacked the dorsal 



he t'^'^r """ *''' '''"^'' '^''- <1> ^^"^ f™™ *« dorsal lip have 

 the power o organize or initiate the construction of the embrvo, and (2) 



org^lttf d'™ V'" ^'l *" '°"^' "P -^ ^''^™'- ^-^f"-; -bryoni 

 organization does not occur ) . 



dermJllTT ^J" '^°''f^'F '''"^ '^^"^'' prospective chorda-meso- 

 oZlZZl" '"'^"'"^'f *" ^"'bryonic organization. Spemann cut 

 g" trulasr "'"' '"'', ™P'™*^'' *em in the ventral sides of other 



occuntt 1 ""TVJ^'" P°''"°" *at chorda-mesoderm normally 

 occup.es, ..e., under the dorsal ectoderm). Two results were observed; 



as a I'r^^f '^^ '"^ "^ impkntation, a small but perfect embr^■o developed 

 ciated organs were present. 



throS JhlehThelernS^^^^^^^ '"I *^ ^'^^^^^T ^^ *^^ -scent-shaped hole 



at the upper or dorsa in onLe M T '"'^'"^ V^'' T^^"«^ ^^ ^^e embryo. Cells 



under th'S par of" he ectocl™ ^Z7Z ?""" ""•^' '"^/^^^ "^ ^^^'^^"^ ^--t'^ 



They become the centrdpart of he m.li ^'"! "'" '° *^" """''"'^^ "^'•^""^ ^^^t^"^- 

 central part of the mesoderm and give rise to notochord and somites. 



