Therian mammals evolved changes in a number of systems
which included additional areas of the cerebrum.
THE NERVOUS SYSTEM
After monotremes, the spinal cord does not extend into sacral region.
In the brains of therian mammals, the nuclei of the dorsal thalamus are
more separated. Therian mammals possess cingulate and subcortical cortices,
dorsal column nuclei, and parabrachial nuclei. The pretectum is more highly
differentiated. The cerebellar hemispheres increased in size.
Therian mammals possess a presylvian sulcus of the cerebrum and share
a secondary somatosensory area S2 and a central somatic sensory area.
The cerebral cortex is composed of 6 layers of cells. Therians modified
the dorsal cochlear nucleus to form an outer molecular layer, a layer
of granule and fusiform cells, and a core of mixed cell types.
The external cuneate nucleus was separated from the cuneate-gracile nuclear
complex. The lagena and ramulus lagenae have been lost. There was an increase
in the size of the ventral cochlear connections in the medulla while dorsal
Theirans developed a distinct monolayer of mitral cells in olfactory bulb
and optic tract fibers projecting to the anterior colliculus arrive at
a deep layer rather than a superficial layer. The tympanic membrane was
THE LYMPHATIC SYSTEM
In therians, the spleen became more compact . Sequence comparisons indicate
that MHC Class Ic genes diverged before the separation of therian mammals.
The class II gene families are only known in therian mammals.
THE CARDIOVASCULAR SYSTEM
Although the embryonic external carotid artery is small and the stapedial
artery is large, as in the condition in non-therian adult vertebrates,
the external carotid grows into the branches of the stapedial artery which
is subsequently reduced or absent. Therians developed umbilical arteries
and the umbilical vein which empties into both the liver and the hepatic
vein. The foramen ovale diverts most blood from lungs during fetal development.
The ancestral ß globin gene had duplicated to produce an proto-eta
gene and a proto-beta gene prior to the separation of the lineages of
therian mammals. Therians evolved an expression of globin after embryonic
development and globin during embryonic development.
THE RESPIRATORY SYSTEM
Marsupial lungs retain reptilian characteristics at birth ( a separate
capillary in each air space, cuboidal epithelia lining alveoli) and are
later modified in a mammalian fashion.