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Biol Bull 80: 185-193. (April 1941)
© 1941 Marine Biological Laboratory
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SIZE AND MORPHOGENESIS IN THE BUD OF BOTRYLLUS

N. J. BERRILL 1

1 The Department of Zoölogy, McGill University, Montreal

In successive bud generations within a colony there is a progressive increase in the size of the bud rudiments and zoöids subsequently developing from them. Dealing with material derived from a single fertilized egg, it is possible to determine the relationship or importance of rudiment size to morphogenesis. Development in every case is simple and direct. It consists of the growth of the rudiment to a maximal disc stage, the conversion of the disc into a sphere, the subdivision of the sphere or vesicle into unit regions, the whole process being accompanied and conditioned by expansion or growth of tissue.

The maximal disc is a precise relative stage. The larger discs, typical of late and mature generations, may be seven times as large in area as those of early generations.

Whatever the size of the maximal disc, succeeding stages bear to it definite growth ratios. A certain percentage expansion or growth of the disc tissue is correlated with a specific developmental stage, whatever the absolute sizes may be. Absolute size must be determined during the initial phase of development before the disc exhibits any tendency to transform into a vesicle.

Ultimate size being thus initially determined, there is variability in the following expressions of size. All organs and regions vary in absolute size, maintaining their proportionate dimensions relative to the whole. Multiple structures, such as stigmata, vary little in absolute size for a given stage but vary in number in proportion to tissue area. The relatively massive gonads, fully formed only in the largest series, are partially or completely inhibited in the smaller.

Gonad formation is essentially a serial separation of the various components during a short phase of development, lasting from the open hemisphere stage to the expanded closed vesicle stage. If the size of the whole permits separation of each component as discrete cells at the proper time for separation, maximal mature gonads will be formed and develop. If size is so reduced that the various components cannot be materially separated as cells, separation is inhibited and no gonads will develop at the normal or any other time. With successive increases in size from this last condition an adequate cellular state is reached, at first including the later phases of the gonadial period and progressively including the earlier, so that a series of immature gonads appear in the inverse order of normal maximal development. Prospective mature ova do not appear at a time normal for the appearance of prospective immature ova or for male cells. Gonad components that do not separate at their normal time do not appear at all.




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