In 1864, Haeckel proposed recapitulation theory or biogenetic law. According to this, ontogeny repeats phylogeny, ie, during developmental stages an animal passes through ancestral adult stages.
Von Baer is known as "Father of Modem Embryology". He also discovered ovum of mammals.
Weismann gave theory of germplasm, which states that inheritance of characters from parents to offspring is mediated through germplasm, which continues generation after generation.
Mendel is known as "Father of Genetics". He published "laws of heredity".