EMBRYONIC GROWTH IN THE VIVIPAROUS POECILIID, HETERANDRIA FORMOSA

¹ From the Biological Laboratories, Harvard University, Cambridge

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The fertilized egg of the viviparous poeciliid Heterandria formosa is minute and is made up almost entirely of a single large oil globule. At least 70 per cent of the original volume 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 increase in wet and dry weight of the embryos at the expense of the mother has been determined by obtaining both wet and dry weights at various stages of development. The dry weight of the embryo increases from 0.017 milligrams at the time of fertilization to 6.8 milligrams at the time of parturition. The parcentage dry weight remains constant at 13.2 per cent after the pseudo-placental association is established.

Observations of living females suggested that under constant optimal conditions the broods of a single female tend to be evenly spaced in time. Upon this assumption the dry weights 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 approximately constant. Food supply and oxygen supply are discussed as factors which might limit this growth. Embryonic growth in Heterandria is compared with that in oviparous fishes.