Embryo

An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of fertilization through sexual reproduction until birth, hatching, or germination.

In humans, an embryo is generally considered to be between the first and the eighth week of development after fertilization, and from then it is instead called a fetus. While most organ systems develop from the third week to the eighth, embryonic life begins at conception.

The development of the embryo is called embryogenesis. In organisms that reproduce sexually, once a sperm fertilizes an egg cell, the result is a cell called the zygote, which possesses half the DNA of each of its two parents. In plants, animals, and some protists, the zygote will begin to divide by mitosis to produce a multicellular organism. The result of this process is an embryo.

Etymology

First attested in English in the mid-14c., the word embryon derives from Medieval Latin embryo, itself from Greek ἔμβρυον (embruon), lit. "young one", which is the neuter of ἔμβρυος (embruos), lit. "growing in", from ἐν (en), "in" and βρύω (bruō), "swell, be full"; the proper Latinized form of the Greek term would be embryum.

Rail transport

Rail transport is a means of conveyance of passengers and goods, by way of wheeled vehicles running on rails.

It is also commonly referred to as train transport. In contrast to road transport, where vehicles run on a prepared flat surface, rail vehicles are also directionally guided by the tracks on which they run. Track usually consists of steel rails, installed on ties (sleepers) and ballast, on which the rolling stock, usually fitted with metal wheels, moves. Other variations are also possible, such as slab track where the rails are fastened to a concrete foundation resting on a prepared subsurface.

Rolling stock in railway transport systems generally has lower frictional resistance when compared with highway vehicles and the passenger and freight cars (carriages and wagons) can be coupled into longer trains. The operation is carried out by a railway company, providing transport between train stations or freight customer facilities. Power is provided by locomotives which either draw electric power from a railway electrification system or produce their own power, usually by diesel engines. Most tracks are accompanied by a signalling system. Railways are a safe land transport system when compared to other forms of transport. Railway transport is capable of high levels of passenger and cargo utilization and energy efficiency, but is often less flexible and more capital-intensive than highway transport is, when lower traffic levels are considered.