Mammals stages

Fig. 4.2 (c) to (d), Early and late embryonic stages in mammals, invagination and separation of AOS from MOS (from Farbmann, 1992). 

The balance between excess and insufficient zinc is important. Zinc deficiency occurs in many species of plants and animals, with severe adverse effects on all stages of growth, development, reproduction, and survival. In humans, zinc deficiency is associated with delayed sexual maturation in adolescent males poor growth in children impaired growth of hair, skin, and bones disrupted Vitamin A metabolism and abnormal taste acuity, hormone metabolism, and immune function. Severe zinc deficiency effects in mammals are usually prevented by diets containing >30 mg Zn/kg DW ration. Zinc deficiency effects are reported in aquatic organisms at nominal concentrations between 0.65 and 6.5 pg Zn/L of medium, and in piscine diets at <15 mg Zn/kg FW ration. Avian diets should contain >25 mg Zn/kg DW ration for prevention of zinc deficiency effects, and <178 mg Zn/kg DW for prevention of marginal sublethal effects. 

Mammal-derived TEFs underestimate the potency of planar PCB mixtures in fish (Newsted et al. 1995). TEF values of non-ort/zo-PCB congeners based on mortality of rainbow trout in early life stages are as much as 1000 times lower than TEFs proposed for human risk assessment (Walker... 

It will be worthwhile, I believe, to consider from this standpoint a field of investigation in which the author has been active, namely, the vitamins. The chemical make-up of the vitamins has been determined by first isolating them in pure form. This has usually involved the use of some test organism which may be a fowl or mammal, a yeast, or a bacteria, but the test organism used is immaterial except for convenience. In order to get test responses as uniform and dependable as possible, it is desirable to have organisms which lack, as far as possible, variation. At this stage of the investigation variation is an enemy. 

The extinction of the dinosaurs had one enduring consequence for us mammals we took over as important players among the living organisms on Earth. While the dinosaurs reigned, the mammals were bit players on the stage of life. If dinosaurs had survived, mammals might have continued to be of minor importance on Earth. 

For birds, insects, and reptiles, which have an egg stage during development, so that water availability is severely restricted, the synthesis of a highly soluble excretory product would have serious osmotic consequences therefore most of the ammonia is converted to the virtually insoluble uric acid (urate). This product can be safely retained in the egg or excreted as a slurry of fine crystals by the adult. In birds that nest colonially this can accumulate in massive amounts on islands off the coast of Peru cormorants have deposited so much that this guano (hence the name guanine) is collected for use as a fertiliser. Uric acid is less effective as an excretory product, since it has a lower nitrogen content than urea (33%) and is more expensive to synthesise (2.25 molecules ATP per atom of nitrogen). Mammals do produce uric acid but as a product of purine catabolism (see above). 

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