BIOL 303 – Study Question – Lecture Set 1
Gastrulation and germ layers
1. What do we mean when we refer to the “development” of an organism?
Development is the process of progressive and continuous change that generates a complex
multicellular organism from a single cell. Development occurs throughout embryogenesis,
maturation to the adult form, and continues into senescence.
Gamete: A specialized haploid male or female reproductive cell through which sexually
reproducing parents pass chromosomes to their offspring; a sperm or an egg.
Zygote: A fertilized egg with a diploid chromosomal complement in its zygote nucleus
generated by fusion of the haploid male and female pronuclei.
Pronuclei: The haploid nucleus of a sperm or egg before fusion of the nuclei in fertilization.
Cleavage: A series of rapid mitotic cell divisions following fertilization in many early embryos;
cleavage divides the embryo without increasing its mass.
Blastomere: a type of cell produced by cleavage (cell division) of the zygote after fertilization
and is an essential part of blastula formation.
Morula: Vertebrate embryo of 16–64 cells; precedes the blastula or blastocyst stage.
Blastula: Early-stage embryo consisting of a sphere of cells surrounding an inner fluid-filled
cavity, the blastocoel.
Blastocoel: The fluid-filled cavity of a blastula.
Gastrulation: A process involving movement of the blastomeres of the embryo relative to one
another resulting in the formation of the three germ layers of the embryo.
Germ Layer: One of the three layers of the vertebrate embryo generated by the process of
gastrulation that will form all of the tissues of the body except for the germ cells.
Triploblastic: Three germ layers
Diploblastic: Two germ layers
Ectoderm: The cells that remain on either the outside or dorsal surface of the embryo following
gastrulation. Of the 3 germ layers, the ectoderm is the one that forms the nervous system from
the neural tube and neural crest and also generates the epidermis covering the embryo.
Mesoderm: The middle of the three embryonic germ layers, lying between the ectoderm and
the endoderm. The mesoderm gives rise to muscles and skeleton; connective tissue; the
reproductive organs; and to kidneys, blood, and most of the cardiovascular tissue.
Endoderm: The innermost germ layer; forms the epithelial lining of the respiratory tract, the
gastrointestinal tract, and the accessory organs (e.g., liver, pancreas) of the digestive tract.
Organogenesis: Interactions between, and rearrangement of, cells of the three germ layers to
produce tissues and organs. Germ cells: A group of cells set aside from the somatic cells that form the rest of the embryo for
reproductive function. Consists of the cells of the gonads (ovary and testis) that undergo
meiotic cell divisions to generate the gametes.
Somatic cells: Cells that form the body; all cells in the organism that are not germ cells.
3. What is holoblastic cle