Complex sentence: While incipient transport of nutrients appears to be common in all viviparous species, some viviparous species receive substantial amounts of nutrients via the placenta (present in therians, some skinks and some fish).
Split into simpler sentences: While incipient transport of nutrients appears to be common in all viviparous species. Some viviparous species receive substantial amounts of nutrients via the placenta (present in therians, some skinks and some fish).