The ovary is formed of (1) a germogen containing at its upper end nucleated protoplasm and lower down germinal cells in groups of four; (2) of a portion formed of successive chambers in each of which there is a row of four germinal cells. Of the four cells only the third develops into an ovum; the remainder are used as pabulum. This is the mode of development in the summer. In the winter the sacrifice of a larger number of germinal cells is required for the development of the ova; and an ovum is produced only in the alternate chambers. In the chambers where an ovum will not be formed an epithelial investment becomes first established round the four germinal cells. The four cells then coalesce, and form a spherical ball of protoplasm from which portions are budded off and absorbed by the investing epithelial cells, which at the same time lose their nuclei. When the whole of the central ball is thus absorbed by the epithelial cells, the latter become used by the winter ovum as food. The winter ovum at its full development is formed of a central mass of food-yolk and superficial layer of protoplasm. CHORDATA. _Urochorda._ (Tunicata.) (58) A. Kowalevsky. "Weitere Studien ü. d. Entwicklung d. Ascidien." _Archiv f. mikr. Anat._, Vol. VII. 1871. (59) A. Kowalevsky. "Ueber Entwicklungsgeschichte d. Pyrosoma." _Arch. f. mikr. Anat._, Vol. XI. 1875. (60) Kupffer. "Stammverwandtschaft zwischen Ascidien u. Wirbelthieren." _Arch. f. mikr. Anat._, Vol. VI. 1870. (61) Giard. "Études critiques des travaux, etc." _Archives Zool. expériment._, Vol. I. 1872. (62) C. Semper. "Ueber die Entstehung, etc." _Arbeiten a. d. zool.-zoot. Institut Würzburg_, Bd. II. 1875. _Cephalochorda._ (63) P. Langerhans. "Z. Anatomie d. Amphioxus lanceolatus," pp. 330-3. _Archiv f. mikr. Anat._, Vol. XII. 1876. _Craniata._ (64) F. M. Balfour. "On the structure and development of the Vertebrate Ovary." _Quart. J. of Micr. Science_, Vol. XVIII. 1878. (65) Th. Eimer. "Untersuchungen ü. d. Eier d. Reptilien." _Archiv f. mikr. Anat._, Vol. VIII. 1872. (66) Pflüger. _Die Eierstöcke d. Säugethiere u. d. Menschen._ Leipzig, 1863. (67) J. Foulis. "On the development of the ova and structure of the ovary in Man and other Mammalia." _Quart. J. of Micr. Science_, Vol. XVI. 1876. (68) J. Foulis. "The development of the ova, etc." _Journal of Anat. and Phys._, Vol. XIII. 1878-9. (69) C. Gegenbaur. "Ueb. d. Bau u. d. Entwicklung d. Wirbelthiereier mit partieller Dottertheilung." Müller's _Archiv_, 1861. (70) Alex. Götte. _Entwicklungsgeschichte d. Unke._ Leipzig, 1875. (71) W. His. _Untersuchungen üb. d. Ei u. d. Eientwicklung bei Knochenfischen._ Leipzig, 1873. (72) A. Kölliker. _Entwicklungsgeschichte d. Menschen u. höherer Thiere._ Leipzig, 1878. (73) J. Müller. "Ueber d. zahlreichen Porenkanäle in d. Eikapsel d. Fische." Müller's _Archiv_, 1854.