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154 case without a parallel in the animal economy, for in the membranes of a developing embryo, the blood originally flows without vessels. With regard to the blood itself, it generally appears quite colourless, or slightly tinged with green. Its motion can be discerned only by means of the globules which it contains, although these, also, are more or less transparent. The globules are very minute, but in this respect they vary much. Those of the caterpillar of the goat-moth are described by Lyonnet as three millions of times smaller than a grain of sand; while those of Agrion Puella, according to Carus, are larger than the globules of the human blood. Their general dimensions may be stated at from the 200th to the 250th part of a line in diameter.

The mechanism of the dorsal vessel, and the manner in which it works, were first rightly understood by Dr. Carus of Dresden, who made his discoveries known to the public in 1826. Much additional light was shortly afterwards thrown on the subject by M. Strauss, in a work published at Paris in 1828. The earlier anatomists had observed its pulsations, and investigated its structure with much care, but were unable to come to any satisfactory conclusion regarding its functions, chiefly from the circumstance of supposing it to have no opening in any part. Lyonnet conceived it to be designed for the purpose of secreting a substance for forming the nerves. Cuvier, also, after a pains-taking investigation, concluded that it