58 University of California Publications in Zoology [Vol. 16 



to this ti'ausition drop is that it requires nearly twice as much serum, 

 which when dealing with very small animals has to be considered. 

 Experiments with tissues from the same embryo in which the tran- 

 sition drop was used in one case and not in the other always showed 

 that the former tissues gave outgrowths more frequent and more 

 vigorous. 



It was nearly always found that the flat drop proved more satis- 

 factory than the deeper hanging drop, since it brings the tissue in 

 contact with the cover-glass, upon which all tissues seem to develop 



Fig. 2. Entire nerve outgrowths of one tissue as viewed with low-power 

 lens. Tissue from anterior dorsal part of embryo. 



best; also it enables the use of the oil-immersion lens which is so 

 essential for detail work. 



Some very large hanging-drop cultures were made by using a 

 common Stender dish. The cover was coated with vaseline on the 

 edge, and in the bottom of the dish was placed moist cotton to keep 

 the serum from drying up. Although more outgrowths were notice- 

 able, no particular advantage resulted, as the thickness of the lid 

 forbids a close study by the high-power lens. 



Accidentally in transference of a tissue which had .sent out some 

 vigorous epithelial outgrowths, bacteria were introduced which in a 

 day developed large iudej>endent colonies. As one of these colonies 



