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56 nerve cells are added to the ventral cord and associated ganglia of Caenorhabditis elegans at about the time of the first larval moult. These cells are produced by the uniform division of 13 neuroblasts followed by a defined pattern of cell deaths. Comparison with the data in the previous paper suggests that there is a relationship between the ancestry of a cell and its function. The significance of programmed cell death is discussed.
In the first of these three papers, White, Southgate, Thomson & Brenner (1976) have shown that the neurones in the ventral cord of Caenorhabditis elegans can be assigned morphologically to distinct classes, and that these classes recur periodically along the length of the cord. The number of cells of each class was found to be invariant within the sample studied, but there were slight variations in their order. We now ask whether any clues to the determination of the various cell classes can be found in the development of the nematode. The life cycle of C. elegans, like that of other nematodes, comprises the egg, four larval stages and the adult. The larval stages are designated L1-L4 and each ends with a moult (table 1). The sexual forms are hermaphrodite (XX) and male (XO); except where otherwise stated, the hermaphrodite form will be considered in this account.
The L1 larva which emerges from the egg is in many respects similar to the adult. The most obvious difference is in the size of the gonad, which contains only four cells. On closer examination, however, a number of other tissues are seen to contain fewer cells than in the adult; these include the gut, the hypodermis, the tail, the ventral cord and the body musculature. It is fortunate for the present study that three-quarters of the ventral cord cells appear after hatching, since the nematode is much more accessible to observation when it is no longer surrounded by the egg case.
Wessing (1953) has already demonstrated an increase in cell number in the gut, hypodermis and ventral cord during the larval development of Rhabditis anomala, a closely related nematode. The following account adds to his data by describing the detailed lineage of some of the late cells. No evidence has been found in C. elegans for the amitotic divisions which he observed in R. anomala.
Web adaptation, Chris Crocker, for Wormatlas, 2008