Development Types of eggs Cell cleavage radial vs. spiral Determinate vs. indeterminate development Blastulae Gastrulation Mesoderm formation Coelom formation Direct vs. indirect development 001_04caddis1.jpg

Types of Eggs Animal pole - tends to become other regions of the embryo 002_04caddis2.jpg Vegetal pole - tends to become digestive system

Types of Eggs Isolecithal - scanty yolk distributed uniformly through egg Centrolecithal - yolk distributed in center around nucleus Telolecithal - yolk concentrated at vegetal pole 002_04caddis2.jpg

Cell cleavage A fertilized egg is a zygote. The zygote splits and the resulting cells are blastomeres. Holoblastic cleavage - cleavage planes pass all the way through the cell. Occurs in isolecithal and weakly telolecithal zygotes. Meroblastic cleavage - cleavage planes do not pass through dense yolk so blastomeres are not separated from each other. 003_04caddis3.jpg

Cell cleavage First cell division is parallel to the animal-vegetal pole (longitudinal division) Second cell division is also parallel to the animal-vegetal pole Third cell division is perpendicular to the animal-vegetal pole (transverse division), sometimes resulting in micromeres and macromeres

Radial vs. spiral cleavage Radial cleavage Spiral cleavage First quartet of cells Second quartet lies directly on top of first quartet Second quartet lies in depressions between first quartet. Top cells are micromeres and bottom cells are macromeres.

Spiral cleavage E.B. Wilson developed a numbering scheme to follow individual cells through spiral cleavage. Macromeres and micromeres are grouped into quartets and numbered according to first quartet ancestor and number of cell divisions. Allows embryologists to follow cell fates and compare development in various taxa.

Determinate vs. indeterminate development Determinate development - fates of early blastomeres (2-4 cell stage) fixed. Embryo will not develop properly if cells are removed. General uniformity in fate of cells for spiral, determinate development: ectoderm from first quartet of cells entoderm from 4a, 4b, 4c, and 4Q cells mesoderm from 4d cell Indeterminate development - cell fate is determined later in development. Blastomeres can be separated early and each separate cell develops normally.

Blastula types Coeloblastula formed from isolecithal eggs that undergo radial cleavage Stereoblastula formed from spiral cleavage Periblastula formed from centrolecithal eggs Discoblastula formed from meroblastic cleavage

Gastrulation Development of a blastula into a multilayered embryo Forms embryonic germ layers which will become different tissues: entoderm becomes gut ectoderm becomes skin and nervous system mesoderm becomes muscles, circulatory system, and internal support system When an embryo has all three of these layers it is triploblastic. If it has only two, it is diploblastic.

Gastrulation Coeloblastulae often gastrulate by invagination. Cells near the vegetal pole grow inward leading to an archenteron and a blastopore. The inner cells are entoderm and the outer cells are ectoderm.

Gastrulation Some coeloblastulae undergo ingression in which cells near the vegetal pole grow to fill the blastocoel. This leads to a solid gastrula called a stereogastrula.

Gastrulation Rarely, cells of the blastula divide to form cells just below them in delamination.

Gastrulation Stereoblastulae that result from holoblastic cleavage generally undergo epiboly. Cells from the animal pole grow over the rest of the blastula. The archenteron occurs secondarily.

Gastrulation Discoblastulae often gastrulate by involution. Cells around the disc proliferate and grow under the disc. The yolk is still present.

Mesoderm formation True mesoderm is derived from entoderm. If a middle layer is derived from ectoderm, it is ectomesoderm and the animal is considered diploblastic. Schizocoely - In organisms with spiral cleavage, the 4d micromere proliferates between the archenteron and the ectoderm to form a solid mass of mesoderm. Enterocoely - In other organisms entoderm pouches off the archenteron and becomes mesoderm. A true coelom is completely surrounded by mesoderm. Some organisms have coeloms that are not surrounded by mesoderm. These are called pseudocoelomates or blastocoelomates.

Indirect development Gamete ---> free living larvae ---> adult Metamorphosis brings huge changes in structure and function. Much mortality at this time. Larvae are induced to metamorphose by cues such as light, temperature, substratum, and conspecifics. Some larvae can delay metamorphosis for days to months. 001_04caddis1.jpg

Indirect development Gamete ---> free living larvae ---> adult Advantages - young do not compete with adults for food or habitat. not much energy put into eggs or care of young. Disadvantages - high levels of mortality (less than 1% typically survive) 001_04caddis1.jpg

Direct development No free-living larvae. Larvae are brooded or encapuslated and hatch as juveniles. Advantages - less mortality. Disadvantages - more energy put into eggs and care of young. young compete with adults but can find different microhabitats. Common in freshwater and terrestrial organisms. Habitats stressful for eggs and larvae

Mixed development Larvae are brooded or encapuslated for some part of their development. Hatch as late-stage free-living larvae.