THE SPACE-TIME PATTERN OF SEGMENT FORMATION IN ARTEMIA SALINA

¹ Department of Zoology, McGill University, Montreal, Canada

1. Growth and the dynamic pattern of segment formation in excysted larvae of Artemia salina have been quantitatively studied. The final shape of Artemia at sexual maturity can be accounted for in terms of initial shape at hatching.

2. In analyzing the pattern of metamerism, the stages of development are gauged by the number of body segments present. Growth during the entire period of segment formation is found to be governed by arithmetical series and sum-of-series relations, implying that growth increments per stage over either initial sizes or initial increases are constant and identical for thoracic, genital, and abdominal segments, respectively. Later transformations of larval shape, resulting in the barrel-shape of the thorax, the presence of a dorsal thoracic curvature, the knobby appearance of the genital segments, and the presence of a straight tapering abdomen, are accounted for analytically on the basis of concepts concerning the age of segments and the time lag involved in segment formation.

3. The presence, absence, and the difference of structure of appendages are shown to be determined, at least in part, by the size of segments when first laid down, and by the time available for appendage rudiments to form appendageal tissues.

4. The time scale employed in the analysis of the segmentation pattern in Artemia is interpreted to be a relative, biological one, and the meaning of the series formulae with regard to this relative scale is illustrated. The notion of "biochronism" is introduced, as a general concept applying to biological events in relative time.