This immunity is transmitted from mother to young, but it only lasts for a short time, about six to eight weeks after birth. Yet this is regarded by Hertwig as an illustration of the transmission of an acquired character, as an acquired modification of the cells of the body, for he explains the immunity on the assumption that all the cells of the body undergo a particular variation due to the influence of the poison, and are thus, to a certain extent, modified in their nature, and that the ovum also undergoes this variation and transmits it to the young animal. The immunization might certainly come under the category of functional modifications, and it might be thought that we have here a case of transmission of such an acquired character. Against this, however, we have to put the fact that _the acquired immunity is not transmitted from the father to the descendants_. Hertwig attempts to explain this by saying that the short duration of the experiments has only allowed the poison to affect the cell-substance (cytoplasm) and not the nucleus, that is, the hereditary substance of the sperm-cells, an assumption which has little probability considering the intimate nutritive relations between the cell-nucleus and the cytoplasm. I should be rather inclined to conclude from the difference in the transmitting power of the sperm and of the ovum, that this 'inheritance of immunity' does not depend, as Hertwig supposes, on a modification of the cells to 'ricin immunity,' but, as Ehrlich and the bacteriologists believe, on the production of so-called 'anti-toxins,' and that these anti-toxins are handed on to the embryo not by the ovum itself, but by the interchange of blood between mother and offspring which lasts throughout the whole embryonic period. It is then self-evident why no transmission of immunity through the father occurs. But it would lead me too far were I to attempt to refute all the attempts that have been made to interpret individual cases as due to the inheritance of acquired characters. I should, however, like to say something as to the theoretical possibility of such an assumption.