The Questions of Developmental Biology

6. In conditional specification, the removal of a blastomere from the embryo can be compensated
for by the other cells' changing their fates. Each cell can potentially give rise to more cell types
than it normally does. This produces a regulative pattern of development wherein cell fates are
determined relatively late. (Examples include frog and mammalian embryos.)
7. In conditional specification, the fate of a cell often depends upon its neighbors ("whom it
meets").
8. In conditional specification, groups of cells can have their fates determined according to a
concentration gradient of morphogen. The cells specified by such a morphogen can constitute a
field.
9. In syncytial specification, the fates of cells can be determined by gradients of morphogens
within the egg cytoplasm.
10. Different cell types can sort themselves into regions by means of cell surface molecules such
as cadherins. These molecules can be critical in patterning cells into tissues and organs.
4. Genes and development: Techniques and ethical issues
"Between the characters that furnish the data for the theory, and the postulated genes, to which the
characters are referred, lies the whole field of embryonic development." Here Thomas Hunt Morgan noted
in 1926 that the only way to get from genotype to phenotype is through developmental processes.
In the early twentieth century, embryology and genetics were not considered separate sciences. They
diverged in the 1920s, when Morgan redefined genetics as the science studying the transmission of traits,
as opposed to embryology, the science studying the expression of traits. Within the past decade, however,
the techniques of molecular biology have effected a rapprochement of embryology and genetics. In fact, the
two fields have become linked to a degree that makes it necessary to discuss molecular genetics early in
this text. Problems in animal development that could not be addressed a decade ago are now being solved
by a set of techniques involving nucleic acid synthesis and hybridization. This chapter seeks to place these
new techniques within the context of the ongoing dialogues between genetics and embryology
The Embryological Origins of the Gene Theory
Nucleus or cytoplasm: Which controls heredity?
Mendel called them bildungsfähigen Elemente, "form-building elements"; we call them
genes. 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 (see Chapter 3). 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 (Figure 4.1), 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, p. 262 declared that the processes of meiosis,