Hyman, Libbie Henrietta. (1916). An Analysis of the Process of Regeneration in Certain Microdrilous Oligochaetes. Journal of experimental zoology. 20(2): 99-163.
416582
Hyman, Libbie Henrietta
1916
An Analysis of the Process of Regeneration in Certain Microdrilous Oligochaetes
This is the journal version of Hyman's Ph.D. thesis
Summary: "1. A gradient in rate of metabolism is demonstrated·in the oligochaetes. 2. In the primary form of the gradient, the rate of metabolism is highest at the head and decreases along the antero-posterior axis. Among the oligochaetes this primary gradient is found only in Aeolosoma, and the zooids of the naids. The primary gradient is an integrative gradient. · 3. In the other oligochaetes examined a posterior region of increased metabolic rate exists, and constitutes a secondary gradient superposed upon the primary gradient. The secondary gradient runs in the reversed direction from the primary; it results from the characteristic method of growth of annelids by continuous formation of new segments posteriorly, and is not integrative in character: 4. In Dero limosa, the secondary gradient involves the posterior third of the body; in Lumbriculus inconstans, it includes the posterior half of the body or more; and in the tubificids; it includes all of the body except the first five to fifteen segments. 5. ·In zooid formation, the gradient of the zooid gradually becomes independent of the gradient of the parent animal and is of the primary form. Owing to the ·processes of growth and dediff erentiation involved in zooid formation the rate of metabolism of the fully developed zooid is higher than that of the parent; i.e., rejuvenescence results from asexual reproduction. 6. In oligochaetes a certain number of the most anterior segments are differentiated as a head. 7. In regeneration, the head and tail are replaced by outgrowth, the other parts by reorganization of the old tissue. No matter how many anterior segments are removed, only the typical number of head segments is, in general, replaced. 8. The head of oligochaetes will not regenerate a tail unless a certain number of trunk segments are included with it; nor will the end of the tail regenerate a head unless of a certain minimum size, Explanations of these facts are suggested. 9. In Dero limosa, any part. of the body, whether long or short, regenerates a normal worm (with exceptions noted in.8) .. 10. In Lumbriculus inconstans, any part of the body, if of sufficient length, regenerates a normal worm. Short pieces show progressive inhibition of head formation along the axis; they give rise to anterior structures showing all gradations between a normal head and a normal tail. Normal posterior regeneration occurs at any level, and with any size of piece, but the number of segments regenerated decreases along the anteroposterior axis. 11. In Tubifex, head formation ceases at about the level of the fifteenth anterior segrnent, and, in Limnodrilus, at the level of the seventh segment, regardless of size of piece. Tail formation occurs at any level. · 12. In Lumbriculus inconstans, it is determined, whether or not the head shall be normal within twenty to twenty-five hours after the pieces are cut. 13. The gradient of an axial series of pieces is not the same as that of a whole worm, because cutting stimulates. This stimulation is greater the shorter the piece and the lower its previous rate of metabolism. This stimulation is temporary, the time of its duration varying with the different species, and is followed by a depression . .15. In Lumbriculus inconstans, short pieces from anterior regions are not much stimulated by section, and depression sets in within the time required for head determination. These pieces produce a high percentage of normal heads. Short posterior pieces are stimulated much more, and the stimulation lasts for a longer period of time, as long as the time required for head determination; these pieces produce a low percentage of normal heads, and a high percentage of inhibited structures. 16. The rate of metabolism of the piece during the time when the head is determined is, therefore, the important factor in anterior regeneration in short pieces. If the rate be high, then the region of new tissue which is to form the head is prevented from attaining the degree of independence and isolation necessary for normal head formation. Head formation will be inhibited in· proportion to the metabolic rate of the old piece. On the other hand, if the metabolic rate of the old piece is low, then the new tissue suffers no inhibition and gives rise to a normal head. The dynamic relations set up between the old and the new tissue after cutting determine the character of the head, or whether a head shall form at all, or whether a tail shall form. 17. In long pieces, the dynamic· factors are unimportant as the primary gradient determines that the cells at any anterior level are always more independent than those of a more posterior level. Therefore, normal heads are always formed on long pieces. 18. Experimental proof of the above conception is presented. If the rate of metabolism of the piece be depressed by means of cyanide, then the percentage of normal heads is increased; if the rate of the new tissue be decreased, then the percentage of normal heads decreases. "
