ABSTRACT
I. INTRODUCTION Beginning with Abraham Trembley's fascination upon his discovery of polyps with "arms shaped like horns" in the 1740s, the freshwater coelenterate, hydra, has been the object of study for numerous generations of interested observers and scientists alike. Much of what we know about hydra-its general structure, its feeding behavior, its asexual reproduction by budding, and its regenerative powers-was actually documented by Trembley during the 3 years he studied hydra (1). Today, hydra represent one of several useful model systems for studying pattern regulation and cell differentiation. Hydra are easy and inexpensive to grow, and when well-fed they can double their number every 2-3 days. Animals produced by asexual budding are genetically alike. Their overall morphology is very simple. They are small enough to maintain in large numbers yet large enough for excision and grafting manipulations. Whether from small pieces of excised tissue or from a reaggregate of dissociated cells, hydra demonstrate a capacity for regeneration that has amazed observers over the years. They are comprised of less than a dozen cell types, including multipotent stem cell populations that behave similarly to the stem cells of vertebrates. The cellular composition of hydra may be artificially manipulated such that the animals may be maintained without the interstitial stem cell and its derivatives. All these characteristics contribute to the belief that studying morphogenesis in
Figure 1 (A) Adult Hydra olgactis with two buds. Scale bar = 1 mm. (B) Some of the terminology used to describe hydra anatomy.
