Human fetuses have the ancestral condition of two frontal bones which fuse to form the single adult bone.
The occipital region of fossil reptiles and mammal ancestors was composed of many bones which gradually fused to become one single occipital bone.
Below are drawings of the formation of the human occipital bone during fetal development.
In fossil fish, amphibians, and anapsid reptiles, the components of the human temporal bone exist as a number of smaller bones.. The squamosal bone (orange) forms the squamous portion of the temporal bone. The bones which will form the petrous portion of the temporal are not visible here. The quadrate (yellow) will be incorporated into the temporal bone along with another small bone (stapes, not visible here) and two bones of the lower jaw.
In primitive vertebrates, the lower jaw is composed of many bones. In the mammalian lineage, the dentary grows larger until it composes the entire lower jaw and the two halves fuse in the midline. The quadrate, articular, and angular—three bones involved in the ancestral jaws—are incorporated into the temporal bone as the malleus, incus (auditory ossicles), and the tympanic ring.
THE DIGESTIVE SYSTEM
The first vertebrates lacked a stomach; the first stomachs developed as specialized regions of intestines. In the human embryo, the stomach begins its development as a tubular region of the GI tract which begins as a generalized tube which then widens and rotates (Sadler, p. 241).
Originally the pancreas is composed of separate dorsal and ventral structures. When the two duct systems fuse, the dorsal duct may be lost or retained as the accessory duct of the pancreas (Sadler, p. 246-7). Additional pancreatic tissue (heterotopic pancreatic tissue) can be found anywhere from the esophagus to the intestines (Sadler, p. 247). Some people have a bifid gall bladder (Sadler, p. 245).
In bony fish and amphibians, the yolk sac is an expansion of the digestive tract which contains yolk. In amniotes, the majority of the yolk sac is extraembryonic lying outside the embryonic body and composing no portion of the adult digestive system. One effect of this change in organization is that the intestines are drawn outside of the embryonic body (they are herniated). In the third month of development the herniated intestines normally return to the body (Sadler, p. 250). In 1 in 5,000 births the intestines are herniated while in 1 in 10,000 births, the liver and intestines are herniated (Moore, p. 288). As the intestines return to the body, they can adhere to other structures, as can the cecum (Moore, p. 290).
In some the cloaca
persists. In some, there is an abnormal
division of the cloaca and the anus may empty into the urethra. Most anorectal abnormalities result from an
unequal division of the cloaca (