More Vigorous Cell Groups Direct Development 87 



of the polyp sticks into the sand of the aquarium, the 

 head is reproduced at the aboral pole. 



Sometimes two distinct groups of cells seem to 

 struggle with each other to constitute the central zone 

 of development, and both may attain their object thus 

 producing double monsters. If from the trunk of a 

 hydra one cuts off at the same time both the tail and 

 the head end, and suspends the fragment horizontally 

 in the water it forms a head at both ends. 



In an analogous way, Morgan in his experiment on 

 the regeneration of Planaria maculata once obtained 

 from a fragment which had been cut off by two trans- 

 verse sections, two heads one at the front end, the other 

 at the hind end. 55 



This simultaneous activation of two centers of develop- 

 ment can go on also at the commencement of ontogeny 

 in the cells of the blastula itself. "From causes which 

 are yet beyond our knowledge, there are often produced 

 (in fish eggs) two gastrular invaginations instead of 

 one, at separate points of the blastula. And it depends 

 on the position of these two invaginations, which could 

 also be designated crystallization centres for the further 

 development of the embryos, how the embryonic cells 

 of the germinal disc are then drawn into the process of 

 development, given very definite positions in relation to 

 one another, and utilized for the formation of organs." 56 



The analogous phenomena of heteromorphosis in 

 general can likewise be explained, through the similar, 



"Morgan : Experimental Studies on the Regeneration of Plan- 

 aria maculata. Arch. f. Entwicklungsmech. d. Org. Bd. VII. 2 and 

 Heft, 3. Leipzig, Engelman. Oct. 18, 1898. P. 381, 395. 



56 Oscar Hertwig: Zeit- und Streitfragen der Biologic. Prafor- 

 mation oder Epigenese? P. 60 



