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A very rudimentary skull is developed at the foremost end of their chorda. The brain of the Cyclostoma is merely a very small and comparatively insignificant swelling of the spinal marrow, a simple vesicle at first. It afterwards divides into five successive cerebral vesicles, like the brain of the Gnathostoma.
From its hinder end a couple of vesicles develop the simple tubular rudiments of the right and left lungs. They afterwards increase considerably in size, fill the greater part of the thoracic cavity, and take the heart between them. Even in the frogs we find that the simple sac has developed into a spongy body of peculiar froth-like tissue.
Now, we say that the little vesicles of which this cloud was composed arose from the condensation of water-vapor preexisting in the atmosphere, through reduction of temperature; we show how they assumed the form they present.
The small advance guard of spermatozoa that may have made their way to the ampullæ will undergo a gradual decrease of their nascent activity, as the days go by. On the occasion of the next nocturnal emission the ampullæ will empty along with the seminal vesicles and these spermatozoa pass out.
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