At times all the moving bodies appear to have tails, especially in the semen of a man and a dog; the motion is then the least brisk, and performed with difficulty. If this liquor is suffered to dry, the tails or threads are deprived of motion the first; the anterior extremity continues to vibrate for some time, and then all motion entirely ceases. These substances may be preserved in this state of dryness for a long time: if a small drop of water is mixed therewith, their figure changes, they are reduced into many globules, which sometimes appear to be in motion, as well by their approximation to each other, as by the trepidation and twirling round their centres.
These moving bodies in the seminal liquor of a man, dog, or bitch, so nearly resemble each other, as to admit of mistaking one for the other, especially if they are examined the moment the liquor is drawn from the animal. Those of the rabbit appear smaller and brisker; but these differences proceed more from the different states in which the liquor is at the time of observation, than from the nature of the liquor itself, which ought to be different in different kinds of animals; for example, in that of a man I have seen streaks of thick filaments, (_fig. 3._) and have perceived the moving bodies separate themselves from these filaments from whence they appeared to proceed; but I have never seen any thing like it in the semen of a dog; where, instead of filaments, or separated streaks, it is commonly a mucilage whose texture is more compact, and in which we with difficulty discern any filamentary parts; yet this mucilage gives birth to moving bodies like those in the semen of men.
The motions of these bodies remain a longer time in the liquor of a dog, than in that of a man; from which it is more easy to be certain of the alteration of form above mentioned. The moment the liquor issues from the body of the animal we perceive the animalcules to have tails; in twelve, twenty-four, or thirty-six hours after, we shall find they have lost those tails, and are then no more than ovals in motion, often much brisker than at first.
The moving bodies are always a little below the surface of the liquor. On the surface some large transparent air bubbles, which have no motion, generally appear, though sometimes these bubbles stir and seem to have a progressive motion, but which is nothing more than the agitation of the air. Below the moving substances we often see others much smaller, and which only appear like globules, having no tails, but the greatest number of which are oftentimes in motion. I have also generally remarked, that in the infinite number of globules, in all those liquors, those which are very small, are commonly black, or darker than the rest; and that those which are extremely minute and transparent, have but little or no motion; they appear also to weigh specifically heavier, for they are always the deepest in the liquor.