Ostertagia circumcincta, a parasitic nematode of sheep, known collectively with the O. trifuroata, the smaller trichostrongyles and several other worms of the same genus Ostertagia, as the "brown hair worms of sheep”, has held a rather obscure position in the evaluation of the importance of the sheep and goat parasites. This fact and their small size account for the elusive past of this nematode. The status of economic importance, however, to which O. circumcincta has now risen, not only in this state, but in the western section of the United States, Europe, New Zealand, and Australia, suffices as an explanation tor further researches bearing on the classification, anatomical organization and cytological study of this parasite. This paper deals quite briefly with the anatomy of the female Ostertagia circumcincta, and somewhat more in detail with the changes observed in the development of the germ cells from the primordial stage through the first cleavage division. 1. The primordial germ cells arise at the tip of the germ tube which lies just posterior to the oesophagus. 2. The period of growth following each stage in the development of the egg is accompanied by a fragmentation of the chromatin material before division takes place. 3. Chromatin strands make their first appearance in the secondary oocytes. 4. The chromatin strands condense to form six bivalent chromosomes or tetrads. 5. The six tetrads split to form twelve individual chromosomes then fuse to form six bivalents which are present when the egg enters the seminal receptacle. N=6 2N=12 6. The eggs remain in one-celled stage until they reach the seminal receptacle where fertilization takes place; immediately thereafter, that is to say, in the uterus. They begin their cleavage divisions. / Master of Science