Morphology and chemical analysis of the sheep conceptus from the 13th to the 19th day of pregnancy

Between the 13th and 19th day of pregnancy the sheep conceptus developed into a structure showing considerable differentiation and all the extraembryonic membranes were established. Both length and dried weight of the embryo increased exponentially during this period. A highly significant regression of dried weight on length of embryos was found but measurement of the additional variable, width, did not improve the accuracy of estimating weight from the embryo’s dimensions. The mass of the extraembryonic membranes also increased greatly. The dried weight of the trophoblast increased 90-fold over this period; that of the yolk sac increased 17-fold from day 15 to day 19. The protein content of each of the structures making up the sheep conceptus approached 50% of dried weight, which is similar to the proportion in adult soft tissues. The contribution of glycogen to dried weight was low in the sheep embryo and embryonic membranes when compared with estimates in the mouse blastocyst. However, at about the time of implantation the level of this polymer in the embryo was high compared with that in adult soft tissues and approached the level found in adult muscle. Concentrations of DNA and RNA in the sheep conceptus are much higher than the levels in most adult soft tissues and probably reflect higher synthetic rates and a smaller cytoplasmic volume in the embryonic cells