produced eggs of the vertebrate pattern; but the color of our skin and hair and eyes, our sex, stature, and mental peculiarities were determined by the sperm as well as by the egg from which we came. There is evidence that the chromosomes of the egg and sperm are the seat of the differential factors or determiners for Mendelian characters, while the general polarity, symmetry and pattern of the embryo are determined by the cytoplasm of the egg.
It will be observed that the correlation between chromosomes and adult characters is different in kind from that between the cytoplasm of the egg and the adult characters; in the latter case polarity, symmetry and pattern are of the same kind in the egg and in the adult, and the correspondence is comparatively close; in the latter there is no correspondence in kind between the chromosomal peculiarities and the peculiarities of the adult. This fact might suggest that the chromosomal organization may be more fundamental than that of the cytoplasm. There are reasons for believing that many substances of the cell are formed by the interaction of nucleus and cytoplasm, and most probably the chromosomes are an important factor in this process. But in no case is the cytoplasm a negligible factor—in no case does it serve merely as food for the chromosomes. The entire cell, nucleus and cytoplasm, is concerned in heredity and differentiation.
D. The Mechanism of Development
Development consists in the transformation of the oosperm into the adult. What is the mechanism by which this transformation is effected? There is progressive differentiation of the germ into the developed organism, but by what process is this differentiation accomplished?
Many different processes are concerned in embryonic differentiation. From the standpoint of the cell the most important of these are (1) the formation of different kinds of substances in cells, (2) the localization and isolation of these substances, (3) the transformation of these substances into the various structures which are characteristic of the different kinds of tissue cells. We shall here describe only the first and second of these processes which are of more general interest than the last.
1. The Formation of Different Substances in Cells
Differentiation consists primarily in the formation of different kinds of protoplasm from the proptoplasm of the germ cells. It is plain that different kinds of protoplasm are present in the two germ cells before they unite in fertilization, but in the course of development the number of these substances and the degree of difference between them greatly increase.
Actual observation shows that by the interaction with one another of
