• The study of embryos, encompasses the study of the development of animals
• Deals with ontogenetic development = individual organism development, rather than
phylogenetic development = evolutionary history of an organism

Stages in Ontogenetic Development
1. Gametogenesis = formation and maturation of sperm and egg (1N = haploid)
2. Fertilization = fusion of sperm and egg to produce diploid (2N) zygote
3. Cleavage = mitotic cell division of early embryo, eventually forming a blastula or blastodisc

Stages in Ontogenetic Development
4. Gastrulation = migration and displacement of single layer of surface cells, still mitotically active, so that three distinct layers are usually formed.
• These layers are the Primary Germ Layers  all tissues and organs in the adult organism may be traced back to these three layers.
– Ectoderm = external layer. Gives rise to skin and nervous system.
– Mesoderm = middle layer. Gives rise to muscles, circulatory system, most of the skeleton, excretory and reproductive systems, etc.
– Endoderm = innermost layer. Gives rise to digestive tract and derivatives (lungs, liver, etc.)

Stages in Ontogenetic Development
5. Organogenesis = continuous masses of cells in the 3 primary germ layers become split into smaller groups of cells  each of which will develop into a specific organ or body part of the animal.
• Early formation = organ rudiments
6. Growth and Differentiation = growth of organ rudiments and acquisition of structure and physiochemical properties allowing them to function as adult structures.

Stages in Ontogenetic Development
• General Rule: In ontogenetic development, general features common to all members of a lineage of animals develop earlier in the embryo than the more specialized or unique features characteristic of specific members of the group.
• EXAMPLE: Features characteristic of all vertebrates (brain and spinal cord, notochord and vertebrae, segmented muscles) appear earlier in development than the features distinguishing various smaller groups (limbs in tetrapods, hair in mammals, feathers in birds), and these appear earlier than characters distinguishing Families,
Genera, and Species.

Stages in Ontogenetic Development
• In terms of evolutionary theory – features of ancient origin appear earlier in development than features of more recent origin.
– This spawned the historic idea that “ontogeny recapitulates phylogeny”
(formulated by von Baer, 1828; popularized by Ernst Haeckel, 1868  became known as the biogenetic law), or that in its embryonic development, the organism passed through previous stages in its evolutionary history.
• However, this “review” is not complete, as many stages in phylogeny are not present in embryonic development, and there are modifications in ontogenetic development that serve as adaptations of the embryo to its environment (e.g., extraembryonic membranes).
– Thus, this idea has been disproved regarding the organism as a whole.
• Despite the incomplete review, there are several anatomical characters or organ systems where this “review” is very important in defining the evolutionary history of organ systems.
– Examples include: vertebrate kidney, pharyngeal arches, aortic arches.

Vertebrate
Kidney
Pharyngeal
Pouches Organ system examples of
“ontogeny recapitulates
phylogeny”
Aortic
Arches

Details of Ontogenetic Development
• Cleavage - During the early stages of cleavage, the cells
(known as blastomeres) show very little growth (e.g., zygote and blastula are about the same size).
• Egg Types are important in determining the nature of the cleavage process
1.
Microlecithal = little yolk, blastomeres equal in size (Mammals,
Amphiouxus)
2.
Mesolecithal = somewhat more yolk (moderate amount).
Blastomeres are unequal in size (Amphibians, lamprey, lungfish).
3.
Macrolecithal = lots of yolk (Reptiles, Birds, Elasmobranchs)
• There are also terms describing the distribution of yolk within the egg:
1.
Oligolecithal = yolk evenly distributed (microlecithal)
2.
Telolecithal = yolk concentrated in one hemisphere (meso- and macrolecithal)

Details of Ontogenetic Development
• Fertilization initiates redistribution of cytoplasmic contents within the zygote, so that gradients of cytoplasmic substances exist.
• This results in polarity of the egg: Animal Pole in relatively clear cytoplasm dorsally, Vegetal
Pole in yolky region ventrally.
• Cleavage results in separation of cytoplasmic substances previously oriented in gradients within the zygote.

• Holoblastic = total cleavage. The entire egg divides, as do successive blastomeres.
– Equal = microlecithal eggs; dividing cells are equal in size
– Unequal = mesolecithal eggs; dividing cells ventrally are larger than those dorsally
• Meroblastic (Discoidal) = division occurs only in a small area at the animal pole (becomes the blastodisc).
• Oligo (Micro)  Holoblastic equal cleavage
(Mammals, Amphioxus)
• Telo (Meso)  Holoblastic unequal (Amphibians)
• Telo (Macro)  Discoidal (Reptiles, Birds,
Elasmobranchs)

• Regular progression of cleavage divisions:
– Vertical plane → produces 2 cells
– Vertical plane, but rotated 90° → 4 cells
– Horizontal plane → 8 cells
• Position of upper cells relative to lower cells during cleavage is important to classification:
– Radial Cleavage = cleavages are symmetrical to the first
(Echinoderms and Chordates – cleavage pattern shows link between these groups, both deuterostomes).
– Spiral Cleavage = cleavages are rotated from thje first
(Annelids, Molluscs, some other invertebrates → separate evolutionary lineage: Protostomes)
• End product of cleavage is the blastula (micro-, meso-) or blastodisc (macro-).

= holoblastic equal
= holoblastic unequal
= discoidal
= holoblastic equal Fig 5.2 – Cleavage stages in chordates

Fig 5.3 – Holoblastic unequal cleavage in the bowfin, Amia
Fig 5.4 – Discoidal cleavage in the Zebrafish, Danio rara

Amphibian Cleavage Video http://www.luc.edu/faculty/wwasser/dev/cleavage.mov

1. Microlecithal (Amphioxus)  hollow sphere
2. Mesolecithal (Amphibian)  hollow sphere, wall is several layers thick.
3. Macrolecithal  blastula forms as a plate, several cell layers thick, on top of the yolk mass
(blastula termed a blastodisc).
• 2 areas of the blastodisc:
– Area opaca = peripheral portion of blastodisc
• attached to the yolk mass
• involved in digestion of yolk and formation of the extraembryonic membranes.
• doesn't contribute to the embryo
– Area pellucida = central part of blastodisc
• becomes lifted off the yolk mass
• forms the actual embryo

4. Microlecithal (mammals)  blastula becomes specialized for placental attachment.
• Early division (cleavage) similar to that in
Amphioxus, but later 2 distinct groups of cells develop:
– Trophoblast = expanded sphere of cells (similar to
Amphioxus blastula)
– Inner Cell Mass = mass of cells lying directly on top of blastocoel. Similar to macrolecithal blastodisc.
• Trophoblast becomes extraembryonic membranes, which form the embryonic side of the placenta.
• Inner Cell Mass develops into the embryo.

Fig 5.5 – Cleavage and blastulae in living mammals

• Ratio of Nuclear (DNA) to Cytoplasmic Material
– very low in zygote
– reaches adult cell levels, essentially without growth, by blastula stage.
• Therefore – the amount of DNA in the embryo increases as division (cleavage) proceeds. Where does this DNA
originate?
– Czihak et al. (1967) - Experiment: radioactive Uridine was given to early sea urchin embryos  some of it becomes incorporated into DNA (indicates conversion of RNA to DNA). Later research showed that conversion was due to the enzyme ribonuclease reductase, present in the zygote.
– So ... one source of increased DNA is from the RNA, present in the cytoplasm of the zygote, which is converted to DNA.
– Grant (1958) - Experiment: 14 C-glycine injected into zygotes  some becomes incorporated into DNA by serving as a precursor in purine synthesis.
– A second source of DNA are precursors (amino acids) present in the zygote (purines = A,G. pyrimidines = C,T,U).

• Protein Synthesis - mostly proteins directly involved in cell multiplication (e.g., histones, tubulin  microtubules, ribonucleotide reductase
{RNA→DNA}).
• Experimental:
– Treat cleaving eggs with puromycin (which inhibits
RNA-dependent protein synthesis)  cleavage stops.
– Treat cleaving eggs with Actinomycin D (which inhibits
RNA production)  cleavage proceeds normally.
• Conclusion: Protein synthesis uses RNA (all three varieties) already present in the zygote.