EMBRYONIC GROWTH IN THE VIVIPAROUS POECILIID, HETERANDRIA FORMOSA

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Abstract
The fertilized egg of the viviparous poeciliid Heterandria formosa is minute and made up almost entirely of a single large oil globule. At least 70% of the original vol. of the oil globule is still present when maternal contribution of nourishment begins. Secretory cells develop in the ovisac wall late in the embryonic period. These together with adaptations previously described permit the mother to contribute nearly all of the raw materials for growth and development of the embryo after the egg has been fertilized. The growth of the embryos at the expense of the mother has been detd. by obtaining both wet and dry wts. at various stages of development. The dry wt. of the embryo increases from 0.017 mgs. at the time of fertilization to 6.8 mgs. at the time of parturition. The % dry wt. remains constant at 13.2% after the pseudo-placental association is established. Observations of living [female][female] suggested that under constant optimal conditions the broods of a single [female] tend to be evenly spaced in time. Upon this assumption the dry wts. of the embryos in each brood of suitable females were plotted against relative age. The resulting curves suggested that the rate of growth after the maternal contribution of nourishment can be detected is approx. constant. Food and O2 supply are discussed as factors which might limit this growth. Embryonic growth in Heterandria is compared with that in oviparous fishes.