Regeneration / by Thomas Hunt Morgan.

  • Thomas Hunt Morgan
Date:
1901
Catalogue details

Licence: In copyright

Credit: Regeneration / by Thomas Hunt Morgan. Source: Wellcome Collection.

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    This is illustrated in the regeneration of the anterior end of the earth- worm ; for example, in the red-banded earthworm, or brandling lobophora fcctida)} If one segment of the anterior end is cut off, one segment is very quickly regenerated (Fig. 2, B)\ if two segments are cut off, two come back (Fig. 2, if three segments are cut off, as many are regenerated (Fig. 2, D^\ if four are cut off, generally four come back (Fig. 2, E)\ when five are cut off, four or five come back (Fig. 2, F); but if six or more are cut off, only four or five are regenerated (Fig. 2, G\ It is found in this case that a limit is soon reached beyond which fewer segments are produced than have been removed. The new segments form the anterior end or head that enlarges to the char- acteristic size; but the missing segments behind the new head are never regenerated, and the worm remains shortened throughout the rest of its life. If the reproductive region has been removed with the anterior part, new reproductive organs are never formed and the worm remains incapable of reproducing itself. This same relation between the number of segments cut off from the anterior end and the number that is regenerated seems to hold good throughout the whole group of annelids, although the maximum number that comes back may be different in different species. Thus in lumbriculus six or seven or even eight new segments come back if more than that number have been removed. If we examine the method of regeneration from the posterior end of a piece of an earthworm, we find that when several or many posterior segments have been removed a new part comes back, com- posed at first of a very few segments. The terminal segment contains the new posterior opening of the digestive tract. New segments are now formed just in front of the terminal segment, the youngest being the one next to the end-segment. The process con- tinues until the full complement of segments is made up (Fig. 3, C, D, E). Comparing these results with those described above for the anterior end, we find, in both cases, that only a few segments are at first formed, but in the posterior regeneration new segments are intercalated near the posterior end. This process of interca- lation is the characteristic way in which many annelids add new seg- ments to the posterior end, as they grow larger and longer. Amongst the flatworms the fresh-water planarians show remark- able powers of regeneration. If the anterior end is cut off at any level, a new head is produced (Fig. 4, C). The new worm is at first too short, z.e. the new head is too near the pharynx, but changes take place in the region behind the new head that lead to the devel- opment of new material in this part. The new head is, in conse- 1 These experiments on the earthworm are in the main taken from my own results (’95) (’97) (’99)-