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CHAPTER 6:
PRINCIPLES OF DEVELOPMENT
surrounded by a resistant
THE REPRODUCTIVE PROCESS capsule (gemmule)
● life springs form from the
reproduction of preexisting life ➔ freshwater sponges
● sexual or asexual reproduction
★ ASEXUAL REPRODUCTION ❖ fragmentation – multicellular animal
breaks into two or more parts, with
➢ production of individuals each fragment capable of
without gametes becoming a complete individual
➢ offspring have the same ➔ anemones and hydroids
genotype as the parent (clone)
➢ bacteria, unicellular ★ SEXUAL REPRODUCTION
eukaryotes, many invertebrate
❖ production of individuals by gametes
phyla
❖ two parents with special germ cells
❖ basic forms that unite to produce a genetically
➢ binary fission – the body of the unique individual
unicellular parent divides by ❖ bisexual/biparental reproduction
mitosis into two approximately ❖ hermaphroditism
equal parts ❖ parthenogenesis
➔ lengthwise – occurs in
➢ bisexual reproduction
flagellate unicellular
➔ production of offspring formed
eukaryotes
by the union of gametes from
➔ transverse – occurs in
two genetically different parents
ociliate unicellular
➔ parents are of different sexes,
eukaryotes
male and female
➢ multiple fission - schizogony
➔ dioecious – having separate
➔ the nucleus divides
sexes
repeatedly before division of
➔ female – produces ovum
the cytoplasm, producing
➔ male – produces spermatozoon
many daughter cells
simultaneously ➢ hermaphroditism
➔ parasitic unicellular ➔ condition of having both male
eukaryotes (sporogony) and female organs in the SAME
➢ budding – unequal division of individual
an organism ➔ monoecious
➔ a new individual arises as an ➔ some fertilize themselves, others
outgrowth from its parent, exchange germ cells with
develops organs, and then another member of the same
detaches. species
➔ prominent in cnidarians ➔ sequential hermaphrodite – a
➢ gemmulation – formation of a genetically programmed sex
new individual from an change occurs within an
aggregation of cells individual organism.
➢ parthogenesis - “virgin origin
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➔ development of an embryo from brownbanded bamboo shar (
an unfertilized egg or one in Chiloscyllium puntatum)
which the male or female nuclei ❖ ovoviviparous – “egg-live-birth”
fail to unite following fertilization ➢ fertilized eggs remain in the oviduct or
➔ ameiotic/diploid uterus while they develop, with
➔ parthenogenesis – no meiosis embryos deriving all their nourishment
occurs from yolk stored within the egg
➔ meiotic parthenogenesis – a Example: greenland shark (Somniusus
haploid ovum is formed by microcephalus)
meiosis, and it may or may not ❖ viviparous – “live-birth”
be activated by the influence of ➢ fertilized eggs develop in the oviduct
a male’s sperm or uterus with embryos deriving their
nourishment directly from the mother
★ THE ORIGIN AND MATURATION
Example: blue shark (prionale glauca)
OF GERM CELLS
★ STRUCTURE OF REPRODUCTIVE
❖ germ cells – provide continuity of
SYSTEMS
life between generations
❖ genital ridges – appearance of the ❖ basic components of a
gonads during early embryonic reproductive system
development 1. primary organs – gonads
❖ primordial germ cells – precursors that produce sperm and
of germ cells eggs and sex hormones
2. accessory organs – assist the
★ SEX DETERMINATION
gonads in the formation and
❖ at first, gonads are sexually delivery of gametes, support
indifferent the embryo
❖ in mammalian males, SRY (sex
★ ENDOCRINE EVENTS THAT
determining region Y) is the male
ORCHESTRATE REPRODUCTION
determining gene.
❖ DMRT1 (Doublesex And Mab-3 ➢ reproduction in vertebrates is
Related Transcription Factor 1) usually a seasonal or cyclic activity
❖ classic experiments performed in ➢ sexual reproductive process is
rabbits provide support for the controlled by hormones
idea that the female is the default (hypothalamus)
sex during development ➢ Sex hormones:
1. females – estrogen and
★ GAMETOGENESIS
progesterone
❖ production of mature gametes 2. males - testosterone
❖ spermatogenesis - gametogenesis
in testis Menstrual cycle
❖ oogenesis - gametogenesis in Estrous cycle
ovaries
animals other humans and
than humans and certain primates
★ REPRODUCTIVE PATTERNS certain primates
❖ oviparous – “egg birth” endometrium in endometrium shred
➢ lay their eggs outside their body the absence of in absence of
➢ may be Internal fertilization or pregnancy pregnancy
external fertilization. Example:
sexual activity at sexual activity at
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❖ uterine contractions – decrease in
time of ovulation any point during
cycle progesterone and increase in
estrogen levels
ESTROUS CYCLE ❖ oxytocin and uterine
prostaglandins continue until fetus
● proestus = 9-10 days (non-fertile is expelled.
stage) ❖ relaxin makes the birth process
- bloody discharge can occur or easier by enabling the expansion of
not, varies in color and intensity the pelvis and dilation of cervix.
among individuals ➢ uniparous - having one
offspring at a time
● estrus = 5-9 days (ovulation, fertile ➢ multiparous - having more than
days) one offspring at a time or to a
- discharge often becomes pinking litter
red, but can alo stop at this
point, while female becomes ★ PRINCIPLES OF DEVELOPMENT
receptive to males
❖ Developmental Biology
● diestrus = 2 months (non-fertile ➔ encompasses the emergence of
stage) order and complexity during the
- bloody discharge can occur or development of a new individual
not and stops eventually, varies from a fertilized egg, and the
in color and intensity among control of this process
individuals ❖ Preformation vs Epigenesis
➔ preformation – young animals
● anestrus = 7-9 months (non-fertile were pre-formed in eggs and that
stage) development was simply a matter
- no bleeding, resting stage until of unfolding what was already
female comes into heat again there.
➔ epigenesis – fertilized egg
MENSTRUAL CYCLE contains building material only,
somehow direct by growth and
● 1-7 days. uterus breaks down, differentiation
menstruation occurs
● 8-11 days. the lining of the womb
pre-formation epigenesis
thickens in preparation for the egg
● 12-17 days. ovulation occurs (usually pre-existing assembled
on day 14)
● 18-25 days. if fertilization has not all people who will each person is put
be born have been together by
taken place, the corpus luteum formed contributions of
each parent
★ HORMONES OF HUMAN
PREGNANCY AND BIRTH the homunculus the adult develops
❖ implantation – embedding of unfolds to form the from an
adult amorphous mass
blastocyst in the endometrium
❖ HCG (human chorionic came from Egyptian came from
gonadotropin) – stimulate the alchemy Pythagoras and
release of estrogen and Aristotle
progesterone
❖ birth/ parturition – initiated by the
release of placental CRH (placental
corticotropin releasing hormone)
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★ FERTILIZATION ❖ meroblastic cleavage
➔ partial, cleavage furrows cannot
❖ the union of male and female gametes cut through the heavy
to form a zygote ❖ yolk concentration
❖ accomplish two things: ❖ isolecithal - eggs with very little yolk
that is evenly distributed in the egg
1. Brings haploid genomes from mother
❖ mesolecithal - moderate amount of
and father together in one nucleus
yolk concentrated in the vegetal
2. Activates the egg to begin pole
development ❖ telolecithal - contain an
abundance of yolk that is densely
❖ an egg cell is 200x as large as a concentrated at the vegetal pole of
somatic cell the egg
❖ sperm is approximately 1/50 the size
of a somatic cell + Most animals proceed through
these stages during development:
★ CONTACT RECOGNITION 1. zygote
BETWEEN 2. early cleavage stages
➔ blastula
❖ most marine invertebrates and ➔ establish polarity and body
marine fishes release gametes into axes
the ocean 3. gastrulastion
❖ in sea urchin eggs, sperm fist ➔ establish germ layers
penetrate a jelly layer surrounding 4. body plan (segmentation)
the egg, then contact an egg ➔ in invertebrates, this involves
vitelline envelope. Egg-recognition neurolation
proteins bind to species-specific 5. morphogenesis (organogenesis)
sperm receptors on the egg.
❖ polyspermy – entrance of more
than one sperm
➔ Fast block
➔ Slow block through cortical
reaction
➔ Fertilization membrane
★ CLEAVAGE AND EARLY
DEVELOPMENT
❖ during cleavage, the embryo
divides repeatedly to form
blastomeres until normal somatic
cell size is attained
❖ animal-vegetal axis is visible on the
embryo before cleavage begins
➔ vegetal pole – yolk-rich end
➔ animal pole – mostly cytoplasm,
little yolk
★ YOLK DISTRIBUTION
❖ holoblastic cleavage
➔ cleavage furrows extend
completely throughout the egg
