A study has been made of cultured cells from the root of Daucus carota at defined stages along the route from quiescent cells to the globular embryos known to produce plants. The salient stages were: (i) the quiescent cells in tissue explants exposed only to a basal medium; (ii) cells variously induced to grow by prescribed growth-promoting supplements; (iii) clusters of cells whether attached to a proliferating explant or free in a liquid medium; (iv) stock cultures routinely maintained in an incipiently embryonic condition in the form of small cell clusters; and (v) cultures in which, as pro-embryonic globules, embryogenesis has been released at will. The fine structure of the cells along this morphogenetic route has been investigated and related to the causal and controlled conditions by which the stages in question were achieved. A range of distinctive cytoplasmic inclusions that occur in cells at these specified stages has been described; these inclusions are not virus particles per se and their occurrence and form is an obvious function of the cultural conditions. The morphogenetic propensity of the cells that give rise to embryos is associated with cytoplasm that is notably free of identified inclusions, that are otherwise conspicuous, but this cytoplasm is rich in microtubules and has some other distinguishing characteristics that are described.

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