I Asked This question TOO MANY TIMES! For Biology = Which of the two largest cavities forms earliest in fetal development? Why do you think that cavity, and its contents, develops before other body parts?

the cladogram shown here shows similarities and differences among several classes of vertebrates. anatomically, the four have limb structure as their common ancestor. we might separate the mammals from the other three classes because they a) divergent; do not lay eggs b) identical; are warm-blooded c) varied; a four chambered heart d) homologous; have mammary glands