ORIGIN OF THE MESODERM AND BEHAVIOR OF THE NUCLEOLUS IN REGENERATION INLUMBRICULUS

Abstract

Nuclei and Nucleoli in Uninjured Individuals.1. The nuclei and nucleoli of the hypodermal cells are small except in the growing tail region. Here they are enlarged, especially in the cells on the ventral side which are involved in the formation of the new nerve cord.2. Large nucleoli are present in the cells of the setigerous glands near the growing region of the tail. In old segments they are small.3. The gut nucleoli are small in the first twelve segments. They are larger from this region up to twenty or thirty segments from the posterior end. In these segments, they are again small.4. Double nucleoli are occasionally found in the mid-gut, where large nucleoli are present.Origin of New Tissue in Regeneration.5. Double nucleoli and mitoses are found in the intestine for eleven or twelve segments from the wound. In this same region the nucleoli are considerably enlarged.6. Cell proliferation in the old intestine practically ceases between the sixth and seventh days of regeneration.7. Neoblasts metamorphose and migrate to the wound at the anterior end as well as at the posterior end. At least eight or nine segments furnish these cells, the four or five nearest the wound apparently playing the most important part as observed by Krecker.8. The failure of the muscle and nerve cells of the old part to form the corresponding new structures in regeneration is perhaps due to the fact that the cytoplasm of these cells has become highly modified, thus rendering them less susceptible to activation.9. The spindle-shaped cells in the dorsal portion of the bud cavity at the anterior end are derived from the hypodermis and not from the muscles of the old part.10. In both anterior and posterior regeneration the nuclei and nucleoli increase in size in the ectoderm cells in the immediate vicinity of the wound. This enlargement is no more rapid in one part than in another; it continues longer in the ventral cells so that by the second day it is greater there.11. The metamorphosis of the ectoderm is not in all probability due to the proximity of the neoblasts, as supposed by Krecker, but instead to an independent transformation.12. The cells of the setigerous glands in the new bud have large nuclei and nucleoli.13. A feature common to all the cells which take part in the formation of the tissues in the regenerating bud is the presence of large nuclei and nucleoli.14. The amount of nucleolar material present in a cell seems to be an index of the activity of its nucleus both in cell-metabolism and in preparation for cell division.15. In Lumbriculus there is no evidence of any division or even of a clearly defined constriction in any of the nuclei of the gut which contain two nucleoli.16. The presence of two nucleoli in a single nucleus is not a step in amitosis, as many have supposed, but is due to the increase in nucleolar substance beyond the amount which can exist within that particular nucleus as a single droplet.17. The various tissues seem to be derived in the same manner both in anterior and in posterior regeneration.