676 EMBRYOLOGY ! INVERTEBRATES 



Amphibia, where it is best known) the fundamental organiser 

 is the dorsal part of that region of the blastula which is going 

 to form endoderm and mesoderm, and in particular that part 

 of it which forms the prominent dorsal lip of the blastopore. 

 Wherever these cells are they will form the proper structures, 

 and they induce the cells which surround them to arrange them- 

 selves in the appropriate way to form the other parts of the body. 



.-kp 



A 



B 



Oy-j-. 



C 



Gc- 



cp" 



-<^N 



Fig. 529. — Eggs of the tunicate Styela, showing locahsation of organ-forming 

 substances. — From Huxley and De Beer, after ConkUn. 



A, Before fertili=ation ; B, just after fertilisation, yellow cytoplasm streaming to vegetative pole ; C, later 



stage ; D, left side view just before cleavage ; yellow cytoplasm, small circles ; yolk, stippled ; clear 



cyptoplasm, white. 

 cp, Clear crescent ; Gc, grey crescent ; Gv, germinal vesicle ; Gy, yolk ; kp, clear region derived from GV ; 



yc, yellow crescent ; yp, mitochondria ; yz, cap of yellow cytoplasm at vegetative pole ; $N, male 



pronucleus. 



If the dorsal lip is cut out of a blastula and grafted in at another 

 spot, its cells invaginate at their new position and an embryo 

 is formed which has quite different axes from those it would 

 have had if the operation had not been performed. If the dorsal 

 lip of one blastula is grafted into another which already possesses 

 its own, invagination takes place at two points, and a double 

 monster is produced (Fig. 530). After the cells have been under 

 the influence of one organiser for some time their fate becomes 

 fixed, and cannot be altered bv the introduction of another 



