It is well known that all organic structures, whether animal or vegetable, are composed of cells.  These cells consist of an outside bag inclosing an inner sac, and within that sac there is a dot.  The outer bag is filled with semi-transparent fluid, the inner one with a perfectly transparent fluid, while the dot is dark and distinct.  In the language of our science, the outer envelope is called the Ectoblast, the inner sac the Mesoblast, and the dot the Entoblast.  Although they are peculiarly modified to suit the different organs, these cells never lose this peculiar structure; it may be traced even in the long drawn-out cells of the flesh, which are like mere threads, but yet have their outer and inner sac and their dot,—­at least while forming.

In the Turtle the ovary is made up of such cells, spherical at first, but becoming hexagonal under pressure, when they are more closely packed together.  Between these ovarian cells the egg originates, and is at first a mere granule, so minute, that, when placed under a very high magnifying power, it is but just visible.  This is the incipient egg, and at this stage it differs from the surrounding cells only in being somewhat darker, like a drop of oil, and opaque, instead of transparent and clear like the surrounding cells.  Under the microscope it is found to be composed of two substances only:  namely, oil and albumen.  It increases gradually, and when it has reached a size at which it requires to be magnified one thousand times in order to be distinctly visible, the outside assumes the aspect of a membrane thicker than the interior and forming a coating around it.  This is owing not to an addition from outside, but to a change in the consistency of the substance at the surface, which becomes more closely united, more compact, than the loose mass in the centre.  Presently we perceive a bright, luminous, transparent spot on the upper side of the egg, near the wall or outer membrane.  This is produced by a concentration of the albumen, which now separates from the oil and collects at the upper side of the egg, forming this light spot, called by naturalists the Purkinjean vesicle, after its discoverer, Purkinje.  When this albuminous spot becomes somewhat larger, there arises a little dot in the centre,—­the germinal dot, as it is called.  And now we have a perfect cell-structure, differing from an ordinary cell only in having the inner sac, inclosing the dot, on the side, instead of in the centre.  The outer membrane corresponds to the Ectoblast, or outer cell sac, the Purkinjean vesicle to the Mesoblast, or inner cell sac, while the dot in the centre answers to the Entoblast.  When the Purkinjean vesicle has completed its growth, it bursts and disappears; but the mass contained in it remains in the same region, and retains the same character, though no longer inclosed as before.