48 H ARC ITT. [You I. 



part assumes the typical shape of the whole, being only differ- 

 ent in sice, this is of course clear. But from an inspection of 

 Fig. i it will be evident that the tentacles are regenerated by 

 a process of budding and growth exactly as in Hydra. The 

 same was even more evident in the regeneration of tentacles, 

 manubria, etc., in the Medusae. In every case they originate 

 as minute buds, and become functional only after a consider- 

 able period of growth. 



Whether additional tissue is formed from "a few undiffer- 

 entiated cells" a sort of reserve embryonic tissue, may indeed 

 be doubtful. Still, that there is growth in the ordinary his- 

 togenic sense must be evident in these cases as truly as in 

 that of the regeneration of the tail or limb of the newt. And, 

 indeed, it hardly seems to be more than a verbal quibble 

 whether it be by one or botJi processes. For in either case 

 it simply implies the presence in these organisms of cells or 

 tissues possessing the capacity, shared in common with em- 

 bryonic or undifferentiated tissues, of reparation. 



VI. Historical. 



Experimental work on the Hydrozoa may be said to date 

 from the classical researches of Trembley, published in 1744, 

 upon species of Hydra. He divided specimens into pieces of 

 various sizes and shapes, and from various portions of the 

 body, securing entire polyps from each. 



He turned polyps inside out and had them live and thrive 

 for months. He also grafted portions of one upon another 

 with equal facility and success. 



These researches were later repeated by Baker, who, while 

 confirming some of Trembley's experiments, was not able to do 

 so for all of them. Similar results were had by Rosel von 

 Rosenhoff in 1755, who in addition claimed to have secured 

 entire polyps from fragments of tentacles. 



In 1878 Engelmann again repeated Trembley's experiments, 

 and with results very similar to those of von Rosenhoff. 



Marshall in 1882 was not able to secure successful grafting 

 or eversion, but regeneration of polyps from portions of ten- 



