the long recognized arteries, veins and capillaries, there is another class of blood vessels of great importance. These vessels are called sinusoids and to a certain extent resemble capillaries, but their development and their relation to the parts of the organs in which they occur are entirely different from those of true capillaries. A sinusoid is developed by the subdivision of a single large vessel. It consists of endothelium, but that endothelium rests directly, or almost directly, upon the cells of the organ in which this type of vessel occurs. Capillaries, on the other hand, are developed as small buds from preexisting vessels and are always found in connective tissue. The most important organ in which sinusoids occur is the liver, and the peculiar circulatory arrangements in that organ, which have so long seemed singular and puzzling, have become comprehensible, and have acquired greater significance since the conception 'sinusoid' was established. This newly formed conception has unlocked the mystery of the portal circulation, and has explained the supply of venous blood to the liver. A morphological explanation of the portal vein had previously remained impossible.
To the fact that embryology explains many anomalies of the adult structure, we have already referred. Let us leave out of consideration the true monstrosities and confine our attention for the present to the anomalies, which are due to arrest of development. These are comparatively frequent, and many of them are so definite we may fairly call them typical. Such, for instance, is the preservation in the adult of the foramen ovale between the auricles of the heart, or of the open ductus arteriosus by which blood of the pulmonary and body circulation may mingle. In case of the veins also an arrangement of vessels is often found in the adult which is due to the persistence of a truly embryonic condition. The urogenital system seems to be peculiarly subject to arrests of development. When it starts the rudiments for both sexes are complete and the two sexes become differentiated largely by the obliteration in the individual of one sex of those structures which are characteristic of the other, but not infrequently it happens that this law of suppression is disturbed, and we then get very interesting and significant anomalies with which the physician often has practically to deal. Such cases do not produce a true hermaphroditism. That is a condition which apparently may occur in the human species, but is of the utmost rarity. Among other of the most frequent and familiar illustrations of arrested development I will mention cleft palate and hair lip. It is quite unnecessary to prolong this list, for it is evident that the anomalies we are considering are of a definite prescribed nature. They are all of practical importance to the physician, and unless he knows something of embryology he can not know what these probable anomalies are. If he does know something of
