The ABH antigens have been mapped out in the tissues of embryos and fetuses, 18 to 125 mm crown to rump length, 6 to 14½ weeks ovulation age.

Both the H and A,B antigens have the same distribution, and their spatial and temporal parallel obtains in intrauterine as well as extrauterine phases of life.

The cell wall antigens are present in their maximal distribution in the youngest specimens available. They outline the endothelium of the cardiovascular system, and the cells of most of the epithelia throughout the body. The exceptions are the liver, the adrenal, and the nervous system, presumed to have lost the epithelial antigens at stages antedating the youngest specimens here described.

The antigens of the stratified epithelia (and of the simple epithelia of the renal collecting tubules), together with the endothelial antigens, are permanent and persist into and throughout adult life. All other cell wall antigens disappear at a time characteristic for each organ. The antigenic recession coincides with recognizable steps of morphological advancement and often with assumption of function by the organ concerned; it is completed at about the end of the first trimester of pregnancy.

The secretion-borne antigens first appear at the 35 to 40 mm stage (8 weeks ovulation age) in the salivary glands and in the stomach, to be followed in a constant sequence by the rest of the gastrointestinal tract, respiratory system, and pancreas. The secretion of these antigens persists throughout life.

The early presence and wide distribution of the cell wall A,B antigens render them likely potential targets for maternal anti-A,B antibodies in heterologous pregnancies; the advent of the water-soluble substances at 8 weeks ovulation age may be providing a buffer shielding the fetal cell wall antigens by mopping up the maternal isoagglutinins.

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