nucleated cells, separated from the cavity of the organ by a delicate structureless membrane. The growth of these cells gives rise to papillary elevations which project into the cavity of the ovary, and eventually become globular

FIG. 33.-Astacus fluviatilis.-A, a lobule of the testis, showing a, acini, springing from b, the ultimate termination of a duct (x 50). B, spermatic cells; a, with an ordinary globular nucleus n; b, with a spindle-shaped nucleus; c, with two similar nuclei; and d, with a nucleus undergoing division (x 600).

bodies attached by short stalks, and invested by the structureless membrane as a membrana propria (fig. 32, m). These are the ovisacs. In the mass of cells which becomes the ovisac, one rapidly increases in size and occupies the centre of the ovisac, while the others

THE OVA AND THE SPERMATOZOA.

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surround it as a peripheral coat (ep.). This central cell is the ovum. Its nucleus enlarges, and becomes what is called the germinal vesicle (g.v.). At the same time numerous small corpuscles, flattened externally and convex internally, appear in it and are the germinal spots (g.s.). The protoplasm of the cell, as it enlarges, becomes granular and opaque, assumes a deep brownishyellow colour, and is thus converted into the yelk or vitellus (v.). As the egg grows, a structureless vitelline membrane is formed between the vitellus and the cells which line the ovisac, and incloses the egg, as in a bag. Finally, the ovisac bursts, and the egg, falling into the cavity of the ovary, makes its way down the oviduct, and sooner or later passes out by its aperture. When they leave the oviduct, the ova are invested by a viscous, transparent substance, which attaches them to the swimmerets of the female, and then sets; thus each egg, inclosed in a tough case, is firmly suspended by a stalk, which, on the one side, is continued into the substance of the case, while, on the other, it is fixed to the swimmeret. The swimmerets are kept constantly in motion, so that the eggs are well supplied with aerated

water.

The testis consists of an immense number of minute spheroidal vesicles (fig. 33, A, a), attached like grapes to the ends of short stalks (b), formed by the ultimate ramifications of the vasa deferentia. The vesicles may, in fact, be regarded as dilatations of the ends and sides

FIG. 34.-Astacus fluviatilis.—A-D, different stages in the development of a spermatozoon from a seminal cell; E, a mature spermatozoon seen from the side; F, the same viewed en face (all × 850); G, a diagrammatic vertical section of the same.

THE PROCESS OF FERTILIZATION.

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of the finest branches of the ducts of the testis. The cavity of each vesicle is filled by the large nucleated cells which line its walls (fig. 33, B), and, as the breeding season approaches, these cells multiply by division. Finally, they undergo some very singular changes of form and internal structure (fig. 34, A-D), each becoming converted into a flattened spheroidal body, about Tooth of an inch in diameter, provided with a number of slender curved rays, which stand out from its sides (fig. 34, E—G). These are the spermatozoa.

The spermatozoa accumulate in the testicular vesicles, and give rise to a milky-looking substance, which traverses the smaller ducts, and eventually fills the vasa deferentia. This substance, however, consists, in addition to the spermatozoa, of a viscid material, secreted by the walls. of the vasa deferentia, which envelopes the spermatozoa, and gives the secretion of the testis the form and the consistency of threads of vermicelli.

The ripening and detachment of both the ova and the spermatozoa take place immediately after the com pletion of ecdysis in the early autumn; and at this time, which is the breeding season, the males seek the females with great avidity, in order to deposit the fertilizing matter contained in the vasa deferentia on the sterna of their hinder thoracic and anterior abdominal somites. There it adheres as a whitish, chalky-looking mass; but the manner in which the contained spermatozoa reach and enter the ova is unknown. The analogy

of what occurs in other animals, however, leaves no doubt that an actual mixture of the male and female elements takes place and constitutes the essential part of the process of impregnation.

Ova to which spermatozoa have had no access, give rise to no progeny; but, in the impregnated ovum, the young crayfish takes its origin in a manner to be described below, when the question of development is dealt with.

FIG. 35.-Astacus fluviatilis.-The last thoracic sternum, seen from behind, with the proximal ends of the appendages, A, in the male, B, in the female, (x 3). am, articular membrane; cxp, coxopodite; st XIV, last thoracic sternum; rd, aperture of vas deferens.