The primates represent a diverse group with the phylogenetic relationships of the var- ious species reflected in their early embryonic development. The members of the Tupaiidae, Lemuridae and Lorisidae show great similarities to the lower mammals and even birds. The Tarsiidae represent an intermediate step and the Platyrrhini and Catarrhini exhibit the greatest similarities to man. During the period of embryogenesis, the dimensions of an embryo are insufficient guide to its age and much more reliable information can be derived from a study of its external form and internal structure. Examination of embryological specimens from a variety of simian species dates back to the early 1900's but much of this material was collected in a haphazard way from animals shot in the wild. Heuser and Streeter (1941) depicted a series of embryos of the rhesus monkey (Macaca mulatta) and correlated their gestational age with external form and size. Streeter (1951) arranged human em- bryos, between 24. +- 1 and 47. +- 1 days of post-ovulation age, into age groups or devel- opmental horizons based on their external form and internal structure. Each horizon was two days apart and numbered XI to XXIII.The transition from embryo to fetus occurred at the end of horizon XXIII and was arbitrarily dermed as the point where bone marrow invaded the cartilaginous precursor in the humerus. The earlier stages I-X have been dermed subsequently, (Heuser and Corner, 1957; O'Rahilly, 1973).
