202 THE ORGANIZER 



pushed out of the wound before they have a chance of becoming adherent. It may be neces- 

 sary to use a glass bridge or Brucke to hold the transplant in position for 30 to 45 inin- 

 utes during the healing process. Such a cover cannot be used longer because it inter- 

 feres with respiration. Observe the healing process and re-examine during 3 days. 



E. TRANSPLANTATION TO THE DORSAL-LIP REGION 



As in "D" above, select two embryos at stage #10 and remove the membranes. The 

 transplantation is to be made from the presun:iptive flank region of the donor to a position 

 just anterior to the dorsal lip of the host. Since the dorsal lip cells move rapidly it is 

 necessary to: 



1. Make the excision from the donor first. 



2. Make the host wound with the donor material nearby, and complete the transfer 

 as quickly as possible. The host wound must be sufficiently anterior to the form- 

 ing dorsal-lip so that the transplant will "take" well before it reaches the level of 

 involution. 



A variation on this procedure is recommended for those students who prove to be pro- 

 ficient in the first part. The donor may be previously stained with Nile blue sulphate 

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

 tached to its new location (i. e. , the dorsal lip region of the host) for about 4 hours, re- 

 move 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. Ex- 

 amine during 3 days for evidence of "organizer" activity acquired by the usually indiffer- 

 ent flank ectoderm temporarily transplanted and located in the dorsal lip environment. 



F. INDUCTIVE CAPACITY OF THE NOTOCHORD OR ARCHENTERIC ROOF 



Dissect living embryos at stages #13 and #14 to locate the notochordal tissue directly 

 ventral to the neural folds. Remove, 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 noto- 

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

 mine how long the notochordal cells will maintain their influential activity. If possible, 

 deternnine 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 original exper- 

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



G. EVOCATION BY INORGANIC SUBSTANCES 



This portion of the exercise constitutes essentially the control experiments for "B" above, 

 the implantation of the living dorsal lip material. 



Obtain the smallest particles of silicon (Okada, 1938) or pieces of cellophane previously 

 soaked in 1/10,000 methylene blue (Waddington, et al 1936) and dried. Insert these 

 small inorgajiic massts into the blastocoel of stages #7 or #8 and observe during 3 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 in- 

 different ectoderm. (See Waddington, 1940.) 



