200 



THE ORGANIZER 



Fig. 1; 

 A 



Fig. 2: 



A. 



E. 



Incision made in order to explont the dorsal lip 

 together with a large amount of presumptive 

 ectoderm. 



The explant fastened to wax substrate. 

 D. Explants after 16 hours of culture. The explant 

 shown in C included a larger amount of entoderm 

 than the one shown in D. 



Diagrams showing successive stages (a-d) in the 

 development of the explants shown in B, C. D. ch. , 

 presumptive chorda (coarse stippling); dl. , dorsal 

 lip; ec, presumptive ectoderm; en., presumptive 

 entoderm (sparsely stippled); g. , tinfoil girder; 

 hm., presumptive head mesoderm (fine stippling); 

 nt. , neural tissue; pr. , presumptive chorda! pro- 

 jection. 



- Various types of explants involving the dorsal 

 lip. 



a - dorsal lip without circumblastoporal material 

 b - dorsal lip with circumblastoporal material 

 c - dorsal lip with circumblastoporal material 



and more presumptive ectoderm 

 d - dorsal lip with more presumptive ectoderm 



and endoderm 

 C, D - Explants after 14 to 16 hours of culture. 



- Explants after 18 to 24 hours of culture. 



G. - Ring embryo produced by removal of the 

 dorsal lip as seen from above (F) and from the 

 side (G). 

 bp. - blastopore 

 bp. gr. - blastoporal groove 

 en. - endoderm 

 hg. - hindgut endoderm 

 nt. - neural tissue 

 proc. - proctodeum 

 tm, - tail mesoderm 

 vs. - vital-stain mark (A, F, G) 



'% 



Fig. 3: 

 A, 



B - Implantation of strips of presumptive ectoderm 

 (ec. im. ) into slits made through the marginal 

 zone on both sides of the dorsal lip. 



D - Two types of ring embryos resulting from the 

 foregoing operation. 



- Insertion of two strips of cellophane (cellg. ) 

 into slits made through the marginal lone on 

 both sides of the dorsal lip. 



- Ring embryo resulting from the foregoing 

 operation. 



ecim. - ectodermal implant 



gr- 

 Imz. 



nt. 

 pr. 



- endoderm 



- groove left by cellophane 



- exposed portion of lateral marginal 

 zone 



- neural tissue 



- presumptive chordal projection 



(Schechtman: 1942 Univ. Calif. Pub. Zool. 51:1) 



