Nucleus or cytoplasm: Which controls heredity?

It is in Mendel's term, however, that we see how closely intertwined were the concepts of inheritance and development in the nineteenth century. Mendel's observations, however, did not indicate where these hereditary elements existed in the cell, or how they came to be expressed. The gene theory that was to become the cornerstone of modern genetics originated from a controversy within the field of physiological embryology. In the late 1800s, a group of scientists began to study the mechanisms by which fertilized eggs give rise to adult organisms. Two young American embryologists, Edmund Beecher Wilson and Thomas Hunt Morgan, became part of this group of "physiological embryologists," and each became a partisan in the controversy over which of the two compartments of the fertilized egg the nucleus or the cytoplasm controls inheritance. Morgan allied himself with those embryologists who thought the control of development lay within the cytoplasm, while Wilson allied himself with Theodor Boveri, one of the biologists who felt that the nucleus contained the instructions for development. In fact, Wilson 1896 declared that the processes of meiosis, mitosis, fertilization, and unicellular regeneration (only from the fragment containing the nucleus) "converge to the conclusion that the chromatin is the most essential element in development."* He did not shrink from the consequences of this belief. Years before the rediscovery of Mendel or the gene theory, Wilson 1895 noted, "Now, chromatin is known to be closely similar, if not identical with, a substance known as nuclein . which analysis shows to be a tolerably definite chemical composed of a nucleic acid (a complex organic acid rich in phosphorus) and albumin. And thus we reach the remarkable conclusion that inheritance may, perhaps, be effected by the physical transmission of a particular chemical compound from parent to offspring."

Some of the major support for the chromosomal hypothesis of inheritance was coming from the embryological studies of Theodor Boveri, a researcher at the Naples Zoological Station. Boveri fertilized sea urchin eggs with large concentrations of their sperm and obtained eggs that had been fertilized by two sperm. At first cleavage, these eggs formed four mitotic poles and divided into four cells instead of two. Boveri then separated the blastomeres and demonstrated that each cell developed abnormally, and in a different way, as a result of each of the cells having different types of chromosomes. Thus, Boveri claimed that each chromosome had an individual nature and controlled different vital processes.

Adding to Boveri's evidence, E. B. Wilson and Nettie Stevens demonstrated a critical correlation between nuclear chromosomes and organismal development: XO or XY embryos became male; XX embryos became female. Here was a nuclear property that correlated with development. Eventually, Morgan began to obtain mutations that correlated with sex and with the X chromosome, and he began to view the genes as being physically linked to one another on the chromosomes. The embryologist Morgan had shown that nuclear chromosomes are responsible for the development of inherited characters.