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The endocrine function of the thymus develops earlier than lymphocytopoietic. Thymalin is produced by epithelial cells in the thymus primordium. It is released into the blood and regulates differentiation of T lymphocytes in the liver, the initial hemopoietic organ. The hormonal and lymphopoietic functions of human thymus are united on weeks 7.5-8 of embryonic life.

Thymocytes of human embryos at the 7–8-week stage have irregularly shaped nuclei with one to three distinct nucleoli, characterized by absence of compact chromatin or heterochromatin. The electron-dense cytoplasm of these cells contains polysomes and a few mitochondria. Receptors for sheep's red blood cells and T-antigen are absent on the surface of the cells. In 11–12-week human embryos, the T-lymphocytes are reduced in size, clumps of heterochromatin appear in their nuclei and receptors for sheep's red blood cells (79%) and T-antigen (60%) appear on the surface of the cells. Later, the quantity of compact chromatin in the nuclei of the thermocytes increases and the cells acquire their definitive properties and structure.

In order to determine the correlations between the activities of the thymus and adrenal glands, a study of the periods of hormone activity was made on 155 fetuses, 9–40 weeks old. The activity of alkaline phosphatases in the intestinal epithelium of the fetus served as a test of the corticoid level in the fetal blood. Small amounts of corticoids appeared in the fetal blood for the first time after the 11–13th week, the amount practically increasing to the 20th week. From the 21–40th weeks, i.e., just before birth, the corticoid content of the blood in both series reached considerable proportions. There is no correlation between the development of the thymus gland and the appearance of corticoids in the fetal blood.

A method of cultivating and fertilizing human ova obtained from ovaries removed at operation is described. A special method of sampling active spermatozoa was used for fertilization. Zygote formation was observed 24–36 h after fertilization of the ripe ova, an embryo of the two-cell stage was formed after 36–40 h, and an embryo at the four-cell stage after 48 h. The number of blastomeres reached 8 after 60–64 h, and after approximately 100–120 h the embryos reached the morula stage. The results provide wide scope for the study of the physiology and pathology of early human embryonic development.