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.®® 
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.” °° 
The analogous phenomena of heteromorphosis in 
general can likewise be explained, through the similar, 
55Morgan: 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. 
5Oscar Hertwig: Zeit- und Streitfragen der Biologie. Prafor- 
mation oder Epigenese? P. 60 
