. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. CILIA. 627 explaining what he meant by the term. Having subsequently discovered that a current existed in the fluid in an opposite direction to that followed by the embryo, he ascribed the mo- tion to an attraction and repulsion exerted by the substance of the embryo on the surround- ing fluid,* more especially at the region of the body where the respiratory organ was afterwards to be developed, and justly conceived that the chief purpose served by it was to renew the water on the respiring surface of the embryo. The att
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. CILIA. 627 explaining what he meant by the term. Having subsequently discovered that a current existed in the fluid in an opposite direction to that followed by the embryo, he ascribed the mo- tion to an attraction and repulsion exerted by the substance of the embryo on the surround- ing fluid,* more especially at the region of the body where the respiratory organ was afterwards to be developed, and justly conceived that the chief purpose served by it was to renew the water on the respiring surface of the embryo. The attraction and repulsion again he supposed to be produced by an oscillatory motion which he perceived on the surface of the embryo. This oscillatory motion, although he describes it as taking place in the substance of the animal, seems to be nothing else than the usual undu- latory play of moving cilia, such as has been already described in other instances,—indeed he himself compares it to the undulation on the arms of polypi. I have distinctly perceived the cilia, though they are very small, in the embryo of the small species of Lymnsea com- mon in this country. It is the one represented in the figure, but considerably magnified. The current takes place along the whole of the sur- face indicated by the small arrows, which also mark its direction, being opposite to that in which the embryo moves. The cilia, though they probably exist over all this surface, were distinctly seen only on the part inclosed be- tween the dotted lines at a ; it required a dou- blet of one-thirty-fifth of an inch focus to make them visible. Appearances similar to those described were discovered by Dr. Grant in the ova of Marine Gasteropoda. In examining the embryos of the Buccinum undatum and Purpura lapillus, which are inclosed in groups within transparent sacs, he was struck with a rapid and incessant motion of the fluid in the sac towards the fore part of the embryo, and he observed that this motio
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