. Biological lectures delivered at the Marine Biological Laboratory of Wood's Holl [sic]. Biology. RE GENERA TION. 2 OI extremely complicated, and we might as well fall back at once on the Bonnet-Weismann theory of preformation. • There is another experiment on planarians that has a direct bearing on Bonnet's hypothesis. If a planarian be almost entirely split in two, leaving the halves connected only at the anterior end (Fig. 4), two new heads may develop at the most anterior end of the cut edges (Fig. 4). Van Duyne, who first carried out an experiment of this sort, found two heads developing
. Biological lectures delivered at the Marine Biological Laboratory of Wood's Holl [sic]. Biology. RE GENERA TION. 2 OI extremely complicated, and we might as well fall back at once on the Bonnet-Weismann theory of preformation. • There is another experiment on planarians that has a direct bearing on Bonnet's hypothesis. If a planarian be almost entirely split in two, leaving the halves connected only at the anterior end (Fig. 4), two new heads may develop at the most anterior end of the cut edges (Fig. 4). Van Duyne, who first carried out an experiment of this sort, found two heads developing, and he interpreted their development as due to a process of heteromorphosis. I have repeated this experi- ment a number of times with the same results, but I think there is a simpler and more obvious way to account for the development of the new heads. They appear at the sides of each half, as they would do were a long piece cut from the side of the body ; but in the latter case the result is not due to heteromorphosis. In the former case the two new heads are, after their for- mation, prevented from being carried forward by the presence of the old head. This interpretation is in har- ^^^ ^ mony with the results of several other experiments. The bearing of this experiment on our present examination is obvious. Two new heads develop, although the old head is present. If the development of the new heads is due to the presence of head-forming substances, as Bonnet supposed, how could they develop as long as the old head is present to use up these substances t The objection might not apply with as much force if the transportation theory did not include the using up of head-forming substances in the old head. Other examples might be cited, but those given above will suffice, I think, to show the improbability of the stuff-trans- portation theory, or, at least, the results show that it cannot be universal in its application to the phenomena of regeneration. The assumption of head
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