The interstitial tissue, besides conveying the blood vessels to the organ, contains many “interstitial cells.” These cells are relatively sparse in the adult bull, and are comparatively delicate, slightly granular, often shuttle-shaped, with a rather small nucleus. Embryologically they are derived from a syncitium arising from the mesothelium of the genital ridge, differentiating out by growth of the cytoplasm. They contain large quantities of fat, and elaborate the internal secretion of the testis. This secretion governs the development of the secondary sexual characters, and has a profound influence on the general body metabolism, and development of the skeleton. The interstitial cells appear early in embryonic life even before there is any differentiation of sex, and their greater relative development in the fetus is indicative of a future male development. In very young embryos, the growth is very rapid, followed, however, by a period of atrophy, during which the seminiferous tubules undergo marked development. Pende (21) states: “There seems to be an inverse relation between the growth of the tubular and interstitial tissues, as one is hypoplastic when the other is in full activity.” From birth to the onset of sexual maturity, which may be called a period of rest for the testicle, the cells are few in number. With the accentuation of the secondary sexual characters, and the beginning of sexual life, these cells again increase in number and activity. The parenchyma of the testis consists for the most part of the seminiferous tubules, which, on account of the courses they take in the different regions, are divided into groups. The peripheral tubules are the much-contorted tubuli contorti. These anastomose to form the much shorter tubuli recti. These in turn anastomose frequently, uniting to form the rete testis. The rete proceeds through the mediastinum to form the efferent ductules which break through the tunica albuginea to form the greater part of the head of the epididymis. The tubuli contorti are the longer and more numerous of the tubules, for it is here that practically all the spermatozoa are produced. The straight tubules are relatively so short that they may be regarded more in the light of the beginning of the system of excretory ducts.