Prior to fusion with the chorion, the extraembryonic mesoderm of the murine (Mus musculus) allantois differentiates with distal-to-proximal polarity into at least two cell lineages: a chorio-adhesive cell lineage called mesothelium, and the endothelium of the umbilical vasculature. How the allantois grows is less clear, but cell proliferation and addition of mesoderm from the underlying primitive streak appear to play important roles. The aim of this study was to analyze growth in the murine allantois. Techniques of histology and microsurgery were used to examine pre-fusion allantoises at nine developmental timepoints that differed by approximately 2 h. Cell counts revealed that allantoic size increased over time. Two hours of exposure to colcemid enhanced mitotic figures, which were used to calculate the relative number of proliferating cells (mitotic index, MI) in pre-fusion allantoises at each developmental timepoint. Cell proliferation was highest in nascent allantoises and showed signs of slowing by two somite pairs. By five to six-somite pairs, when most allantoises are attaching to the chorion, the overall MI decreased significantly. No regional differences in the mitotic index were observed at any developmental stage. Total cell numbers and the mitotic index were then used to discover the extent of streak contribution to pre-fusion allantoises. Cell proliferation and streak activity were highest in nascent allantoises, after which growth occurred predominantly by cell proliferation. Formation of allantoic regenerates by microsurgical removal and culture in intact conceptuses provided independent confirmation that, as the allantois matured, the primitive streak ceased to be a major contributor to its growth. Thus, the allantois grows by both mitosis and addition of mesoderm from the streak. That the periods of highest cell proliferation and streak activity coincided raises intriguing questions concerning their interplay in the control of growth in the murine allantois.