> See also: > - Reference # Embryonic Development Immediately following fertilization, the new multi-cellular organism will consist of a single-celled zygote. The rapid, multiple rounds of cell division that follows is called **cleavage**. ![[Pasted image 20230105015200.png]] During this process, the organism is rapidly increasing its cell count while not increasing its size. Once the embryo has consists of over 100 cells it is known as a **blastula**. - These blastulas are typically a hollow sphere of cells (**the blastoderm**) with a type of fluid/yolk contained within the central area (**the blastocoel**). ![[Pasted image 20230105015124.png|400]] Following the formation of the blastula, a hole will begin to form within the embryo called the **blastocyst**. Cells that are caving inwards are known as the **inner cell mass** while those remaining on the surface are **trophoblasts**. the embryo is the inner cell mass while the outer layer contributes to placenta? - ??? https://openstax.org/books/biology-2e/pages/27-2-features-used-to-classify-animals ## Gastrulation ![[Pasted image 20230105020149.png]] A blastula is simply a cluster of cells. In order for a complex organism to develop it must begin the formation of a **body plan**. **Endoderm** - 1 **Mesoderm** - 1 **Ectoderm** - 1