THE ORGANIZER 215 



A variation on this procedure is reconmiended for those students who prove to he pro- 

 ficient in the first part. The donor may he previously stained with Nile hlue sulphate 

 (1 part in 500,000) so that the transplant will he identifiahle. After it has become at- 

 tached to its new location (i.e., the dorsal lip region of the host) for ahout k hours, 

 remove it (without bothering to protect the host), brush away all host cells with a hair 

 loop, and then implant the stained cells into the blastocoel of another embryo at stage 

 #7- Examine during 3 days for evidence of "organizer" activity acquired by the usually 

 Indifferent flank ectoderm temporarily transplanted and located in the dorsal lip environ- 

 ment. 



F. INDUCTIVE CAPACITY OF THE NOTOCHOBD OB ABCHEHTERIC ROOF 



Dissect living embryos at stages #15 and #1^+ to locate the notochordal tissue direct- 

 ly ventral to the neural folds. Bemove, and clean strips of notochord by means of a hair 

 loop and watchmaker's forceps. Such notochordal tissue may be Implanted into the blas- 

 tocoel ("B") or explanted ("C") to determine organizer or inductive capacity. The 

 notochord is derived from cells involuting over the dorsal-lip and it is of interest to 

 determine how long the notochordal cells will maintain their influential activity. If 

 possible, determine the portion of the notochord used, whether anterior or posterior. 

 Similarly, the archenteric roof may be identified (generally grayish cells) and parts of 

 it may be implanted and explanted to test the duration of inductive capacity. The origin- 

 al experiments of this nature led to the concept of "individuation". (See glossary.) 



G. EVOCATION BY INOBGANIC SUBSTANCES 



This portion of the exercise constitutes essentially the control experilments for "B" 

 above, the implantation of the living dorsal lip material. 



Obtain the smallest particles of silicon (Okada, 1958) or pieces of cellophane pre- 

 viously soaked in l/lO,000 methylene blue (Waddington, et al 1956) and dried. Insert these 

 small inorganic masses into the blastocoel of stages #7 or #8 and observe during 5 days for 

 evidence of Inductions. 



Sterols, saponins, glycogen, cephalin, oestrogenic and carcinogenic substances, dead 

 tissues from a variety of animal sources, and tissue extracts from worms to mammals have 

 been used to successfully cause significant changes in contiguous but otherwise indiffer- 



ent ectoderm. (See Waddington, 19'+0.1 



*************** 



OBSEBVATIONS AND EXPEBIMENTAL DATA : 



The post-operative care of the embryos generally includes returning them to their 

 normal growing medium after the healing of the wounds ?n the operating medium. Specimens 

 should be kept in separate #2 Stenders or finger bowls, properly marked for identifica- 

 tion, and placed at cool temperatures to reduce bacterial growth. 



The maximum duration of observations for these experiments is about k days after the 

 operation. Sketches and photographs at the time of the operation, with similar records 

 at appropriate intervals, and finally, histological confirmation of the macroscopic ef- 

 fects are recommended. The results are qualitative in that no two experiments could pos- 

 sibly be alike, hence complete and accurate records of each specimen are most important. 



(The student should study the Glossary to learn the distinction between organizer . 

 inductor , and evocator as illustrated the above experiments.) 



