Why was developmental biology left out of the "modern synthesis" of evolution as it developed in the 1930s and 1940s?

A) Many developmental biologists believed that evolution progresses by rapid jumps, not slow and gradual change.
B) It was widely felt that development and genetics had to be "properly separated" for either to make any progress.
C) Many developmental biologists focused on "structuralist" explanations of evolution.
D) The genetic mechanisms of development were poorly understood at the time.
E) All of the above.