not with others. Specificity is dependent on chemical
composition, physical forces, and molecular structure
at the binding site.
spectrophotometer An instrument that measures
the intensity of light versus its wavelength.
Spemann, Hans(1869–1941) GermanEmbryologist
Hans Spemann was born on June 27, 1869, in Stuttgart
to the publisher Wilhelm Spemann. From 1878 until
1888 he went to the Eberhard-Ludwig School at
Stuttgart, left school in 1888, and spent a year in his
father’s publishing business.
In 1891 he entered the University of Heidelberg
after spending his military service from 1889 to 1890.
He studied at the University of Munich during the win-
ter of 1893–94, and from the spring of 1894 to the end
of 1908 he worked in the Zoological Institute at the
University of Würzburg.
In 1895 he took his Ph.D. degree in zoology,
botany, and physics, and three years later was qualified
as a lecturer in zoology at the University of Würzburg.
In 1908 he was asked to become professor of zoology
and comparative anatomy at Rostock, and in 1914 he
became associate director of the Kaiser Wilhelm Insti-
tute of Biology at Berlin-Dahlem. In 1919 he was
appointed professor of zoology at the University of
Freiburg-im-Breisgau, a post that he held until he
retired in 1935, then becoming emeritus professor.
Spemann’s work was in experimental embryology,
focusing on the differentiation of embryo cells during
an organism’s development. He was an expert in micro-
surgical technique. He worked on amphibian eggs and
in 1924 discovered that a part of an embryo, when
transplanted to other regions of the embryo, causes a
change in the surrounding tissues. Those parts were
named “organizer center” or “organizer” by him. The
so-called organizer effect was the dominant topic in
embryological research during the 1930s, and for this
discovery he was awarded the Nobel Prize in 1935.
Spemann laid the foundations of the theory of
embryonic induction by organizers, which led to bio-
chemical studies of this process and the ultimate devel-
opment of the modern science of experimental
morphogenesis. He even conducted one of the first
cloning experiments on a salamander. He described his
researches in his book Embryonic Development and
Induction (1938). Spemann died at Freiburg on
September 9, 1941.sperm The small, mobile, haploid gamete that is pro-
duced by sexually reproducing male eukaryotes.
See alsoSEMEN.spermatogenesis The development of sperm cells in
the tubules of the testes from spermatogonium.S phase The time period when DNA is synthesized
(S) in a cell so that new cells forming will have the cor-
rect amount of DNA. One part of a five-part process in
cell division.
See alsoMITOSIS.sphincter A ringlike muscle that surrounds a natural
opening and closes it by contraction.spiders Members of the class Arachnida, order
Araneae, and one of the most feared organisms by
humans. There are more than 30,000 species of spiders
belonging to 105 families. All arachnids have four pairs
of walking legs and fangs (chelicerae) adapted for liq-
uid feeding. They lack jaws or other types of feeding
structures.
All spiders have spinnerets, small fingerlike
appendages at the end of their abdomen, that secrete
silk during all stages of life. The silk is used for captur-
ing prey, rearing young, mobility, making shelter, and
for spinning webs. Spiders mate when the male trans-
fers sperm to the female using pedipalps, specially mod-
ified appendages near the mouth. Some spiders are
poisonous, and all are predators, eating many different
kinds of insects.spina bifida(myelomeningocele) A neural-tube defect,
a birth defect, that results in the failure of the bones of
the spine, usually at the bottom of the spinal cord, to
close during the first month of pregnancy. Paralysis is
the usual condition, along with no control of somespina bifida 311