In a previous paper on Fundulus hybrids2 it was stated that the developmental rhythm of the young embryo was distinctly influenced by the foreign spermatozoon as early as fourteen hours after fertilization. It was also conjectured that the influence of the male cell was operative a t a much earlier period, although i t did not manifest itself in a measurable degree. Attempts were made to test the influence of the foreign sperm upon the early cleavage rhythm, but the results were largely negative owing to the crudeness of the methods employed.
As the writer was convinced that these results needed reexamination, the work was resumed during the summer of 1909. This time the methods proved to be sufficiently refined to suit the case and positive results were obtained. The treatment was statistical in the sense that very large numbers of eggs were examined, and this involved an amount of tedious labor and eye-strain probably out of proportion to the value of the results obtained.
During the earlier attempts much difficulty was experienced in obtaining satisfactory data concerning the rate of cleavage. It seems a simple enough matter, to one who has not made the attempt, to enumerate the 2-and 4-cell stages in a batch of a thousand eggs or more. In reality, however, the difficulties are numer-Contributions from the Zdogical Laboratories, K'niversity of Texas, No. 100.