Abstract
The histochemical distribution of unspecific alkaline phosphatase in two human embryos in Streeter's horizon xii has been presented. The highest activity of the enzyme is observed in the neural tube, the floor plate regions of the mid- and hindbrains and the spinal cord exclusive, the lung bud, the thyroid diverticulum, the cloaca and the primordial germ cells. Those showing moderate reaction are the proximal parts of the neural crests, the auditory vesicle, some epibranchial placodes, major part of the gut, the myo-dermatomes of the somites, some parts of the coelomic epithelium and some of the nephric structures. A specific suppression of the enzymatic reaction is discernible in the floor plate of the neural tube, the peripheral parts of the neural crests, the esophagus, the liver primordium, the sclerotomes and the caudal part of Wolffian duct. In addition, this stage is characterized by the following facts: the formation of the acoustic nerve from the auditory vesicle takes place; the facial and glossopharyngeal placodes are established as the thick, highly phospbatase positive areas of the integumentary ectoderm; the production of the primordial germ cells extends to the coelomic epithelium from the hindgut epithelium; the vascular endothelium becomes phosphatase positive.
