The Reactions of the Vertebrate Embryo to Stimulation etc. 577 



2) Prior to the Silver bath the material must be washed for 

 several hours or batter over night in running water, then passed 

 through several changes of distilled water and finally brought into 

 a 0,75X or \% solution of nitrate of Silver. In warm weather, at 

 the end of 4 days, the tissue will be found to bave assumed a pale 

 café-au-lait tinge. In cooler weather this reaction generally takes 

 place between the 5th or 7th day, depending on the size of the 

 embryos as well as upon certain other condìtions, which are not 

 understood. Of course ali light must be carefully excluded from 

 the specimens as long as they remain in the Silver solution, and if 

 auy metallic deposit occurs upon their surface it is well to rinse 

 them quickly in distilled water and place them in a new solution. 

 For some reason the use of stronger solutions of silver does not give 

 satisfactory results in embryos. 



3) The specimens are again rinsed in distilled water and placed 

 for Va — 2 hours in the dark in the foUowing solution which has 

 been tilteved. 



0,75—1^ Sol. of Silver nitrate .... 20 ce 



Caustic soda (40^ sol.) gtt. 4 



Liq. ammoniaci caust gtt. 10 — 12 



sufficient to dissolve the precipitate. 



The material is left in this solution until it has assumed a dark 

 reddish brown color and is decidedly translucent. If the reddish 

 tinge does not appear it is useless to carry the tissue any further. 

 When removed from this solution the specimens are again passed 

 quickly through two or three changes of distilled water and placed 

 at once in a dish containing 



4) Distilled water, to every 10 ce of which 5 drops of glacial 

 acetic acid bave been added. After a few minutes (5 — 15) the red- 

 dish tinge of the tissue becomes decidedly yellow when it is again 

 quickly rinsed in distilled water and transferred to 



5) the developer, composed of hydrochinone (1^ solution) 20 ce 

 and neut. formol 2 ce and left in this for 12 hours. 



6) For the last time the material is washed in distilled water^ 

 preferably in a small vial, as it is then possible to dehydrate ra- 

 pidly by adding stronger solutions of alcohol. Experience has 

 convinced me that this is a very important step in the process. If^ 

 begìnning with 35^ alcohol, the strength is increased by adding 

 a few drops at a time of a stronger alcohol, the shrinkage in the 

 tissue may be reduced to a minimum. Cedar oil, Chloroform, xylol 



37* 



