T H HE fact that an egg may, under certain conditions produce two or more embryos is perhaps no more remarkable than that an egg will form a single individual. The developmental factors involved in the production of the individual embryo must be the same in both cases. Nevertheless, the discovery that multiple embryos develop from the egg of a given species has served to arouse the interest of biologists. In the zoological literature the term polyembryony has been applied to cases in which two or more individuals develop from a single egg during the course of its early development. The term was first used by botanists, and among plant embryologists is applied to all cases in which multiple embryos are formed in the embryo sac, irrespective of the origin of the embryos. Thus in plants multiple embryos (polyembryony) may arise from two eggs, or from the splitting of one egg, or from any one of the following sources: nucellus integument, synergids, antipodal cells, endosperm cells, suspensor. Animal embryologists use the term in the restricted sense only, that is, it is applied by them to cases in which the several embryos develop from one egg. Some objections have been offered to the term polyembryony, especially in applying it to cases of twinning in animals. But so long as the term is used in a purely descriptive sense, and without implying any particular mode of development, there can be no serious objections to its universal application. Twinning itself must be regarded as the simplest type of polyembryony. That this is true, has been demonstrated in one of the parasitic hymenoptera, to which reference is made below. Three types of polyembryony may be recognized. These are: (i) Experimental polyembryony, or the production of multiple embryos by artificial means; (z) accidental or sporadic polyembryony, or the occasional production of multiple embryos in a species in which development is typically monembryonic; (3) specific polyembryony, or the habitual production of multiple embryos in a given species. Among the first to produce experimentally two or more embryos from the egg was Haeckel ('69). He cut into pieces the blastulae of Crystallodes and obtained from the larger pieces normal larvae. Since then there have been many successful experiments of a similar nature. Among these may be mentioned the work of Wilson ('93), who isolated the blastomeres of the eggs of Amphioxus by shaking, and found that such separated cells were capable of forming complete embryos; the classical experiments of Driesch, and of others, on the eggs of Echinoderms; the studies of Schultze ('95), Herlitzka ('97), and Spemann ('oi, '03) on Amphibian eggs; and more recently, the interesting work of Stockard ('zi) in producing twins and double monsters in the fish egg by lowering the developmental rate. The results of these, and many similar