Development AxolotlChicken

Gametogenesis The formation of gametes occurs within the gonads (ovaries and testes.) Spermatogonia and oogonia undergo mitosis to produce primary spermatocytes and oocytes. Primary gametocytes undergo meiosis twice to produce four spermatozoa or a single ovum.

Fertilization The haploid egg and haploid sperm unite resulting in a diploid zygote.

Cell Cleavage  Rapid divisions increase cell numbers with no increase in size  1 st Cleavage = 2 cells  2 nd Cleavage = 4 cells  3 rd Cleavage = 8 cells  4 th Cleavage = 16 cells  5 th Cleavage = 32 cells (morula)  Hollow ball of cells (blastula) forms with a fluid filled cavity in the center (blastocoel)

Gastrulation  Invagination (denting inward) and involution (rolling inward) of cells producing blastopore.  The 3 primary germ layers are established:  Ectoderm  Mesoderm  Endoderm

Gastrulation in Aquatic Vertebrates  Ectoderm formed as cells from the animal pole surround larger yolk cells which form the endoderm.  Mesoderm formed at an involuting dorsal lip at the blastopore (see models)

Gastrulation in reptiles, birds, and mammals The cells of the blastodisc which are adjacent to the yolk become endoderm. Mesoderm is formed at the involuting primitive streak (an elongated blastopore) on the dorsal surface of the blastodisc.

Germ Layer Development  Ectoderm  Nervous system, cornea and lens of eye, epidermis of skin, epithelial lining of mouth and rectum  Mesoderm  Circulatory system, segmented muscles, skeleton, kidney, gonads, outer covering of internal organs, lining of thoracic and abdominal cavities  Endoderm  Pharynx, liver, pancreas, lining of digestive tract, lining of respiratory tract

Cleavage (2 cells)Cleavage (4 cells)Blastula Early Gastrula Late GastrulaEarly NeurulaEarly Tail BudLate Tail Bud Axolotl Development

Chick Embryo Development 4 day embryo removed from the egg: gY 9 day embryo inside egg: Vk Fertilized egg to a baby chick LTg&feature=related

Candling: Interpreting what you see… Freshly laid egg (air pocket and yolk are visible): if it is over a week old it is not fertile Fertile egg at approximately 5 days old. Notice the faint blood vessels. Opaque egg: the chick has developed to the point that you can’t see through the egg at all. This egg is opaque, but it has a large, off-center air pocket. It could contain a dead-in-shell chick, or it could be an old rotten egg.

Chicken Egg Candling Unfertilized Chicken EggFertilized Chicken Egg

Today’s Tasks Station 1: Model amphibian development –Sketch 1 stage & label layers & other identifiable structures Station 2: Preserved and live frog embryos –1 sketch any stage Label developmental stage and identifiable structures –Discuss questions in your discussion Station 3-4: Live axolotl, frog & killifish embryos –2 sketches of axolotls at different developmental stages –2 sketches of killifish at different developmental stages Label developmental stages, and any of the following you can find: yolk, head, tail, dorsal/ ventral sides, eyes, blood vessels, heart –Discuss questions in your discussion Station 5: Chick Embryos –2 sketches at different developmental stages Refer to description card to label all identifiable parts –Discuss questions in your discussion