Fertilization

day 0-1

fertilization —> zygote forms; sperm pierces the zona pellucida

day 2-3

end of day 3

day 4

differentiation into trophoblast; blastocyst forms

day 7

hypoblast delaminates from inner cell mass

day 8

bilaminar disc (epiblast + hypoblast); amnion and primary yolk sac appear

day 8-12

growth

day 12

extraembryonic mesoderm and extraembryonic coelom form

day 13

connecting stalk and prochordal plate form; oval disc

day 15

primitive streak appears

day 17

cloacal membrane forms; gastrulation —> trilaminar embryo

day 17-21

notochord forms and grows cranial

day 20-21

neurulation (neural folds) —> neural groove —> neural tube; nueral crest appears

zona pellucida

pierced by the sperm at fertilization

blastomere

individual cells produced by cleavage

morula

solid ball of 16 blastomeres

trophoblast

outer layer of cells on the blastocyst; will contribute to the placenta, not the embryo itself

embryoblast (inner cell mass)

inner clump of cells inside the trophoblast sphere; develops into the embryo

blastocyst cavity

fluid-filled cavity inside the sphere of trophoblast cells

blastocyst

germ sac

hypoblast

• the under layer of the bilaminar disc

• contributes to the yolk sac lining and later turns into the endoderm

epiblast

• upper layer of the bilaminar disc

• gives rise to the amnion and later turns into the ectoderm

bilaminar embryo (embryonic disc)

• 2 layer epiblast

• hypoblast disc present

amnion

• membrane that closes the amniotic cavity

• keeps the embryo from drying out

exocoelomic membrane (primary yolk sac)

extraembryonic mesoderm

goo that surrounds the amnion and yolk sac

extraembryonic coelom

small fluid spaces that form in the extraembryonic mesoderm and merge into a larger cavity

connecting stalk

stalk of the extraembryonic mesoderm linking the embryo to the mesoderm lining the trophoblast wall; forerunner of the umbilical cord

prochordal plate

marks for the mouth

primitive streak

site where the cells migrate inward to form the mesoderm during gastrulation

cloacal membrane

marks for the a*us

gastrulation

formation stage where the “gut” layer arrangement first appears

mesenchymal cells

migratory cells extruded from the primitive streak that invade the space between epiblast and hypoblast and replace the hypoblast

cardiogenic area

region where heart development begins

ectoderm

• new name for the epiblast once gastrulation is complete

• outer skin layer

endoderm

new name for hypoblast layer when the cells are replaced my mesenchymal cells; inner skin layer

intraembryonic mesoderm

new middle layer of tissue sitting between the ectoderm and endoderm

trilaminar embryo

has the ectoderm, mesoderm, and endoderm layer

notochord

establishes the embryo’s body axis and signals the ectoderm to form the neural plate

neural fold

neural tube

precursor for the central nervous system

neural groove

neural crest

median sagittal plane

plane that bisects the embryo into 2 symmetrical halves

cranial/caudal

head-ward/tail-ward directions

ectopic pregnancy

implantation outside the uterus