(74) W. H. Ransom. "On the impregnation of the ovum in the Stickleback." _Pro. R. Society_, Vol. VII. 1854. (75) C. Semper. "Das Urogenitalsystem d. Plagiostomen, etc." _Arbeiten a. d. zool.-zoot. Instit. Würzburg_, Vol. II. 1875. [Cf. Ludwig, No. 4, Ed. van Beneden, No. 1, Waldeyer, No. 6, &c.] There are some very obscure points connected with the growth of the ovum of the Tunicata. When quite young the ovum is a naked cell with a central nucleus containing a single large nucleolus. Around it is a flat follicular epithelium enclosed in a membrana propria folliculi. The follicle cells soon become larger and give rise to an envelope round the egg of the nature of a chorion. At the same time they frequently become cubical or even columnar, and filled with numerous vacuoles. During or after the completion of the above changes a number of bodies usually spoken of as test-cells make their appearance in the superficial protoplasm of the egg, which by the time the egg is ripe arrange themselves in many species as a definite layer round the periphery of the ovum. These bodies have received their name from the opinion, now known to be erroneous (Hertwig and Semper), that they eventually migrated into the test or mantle of the embryo which becomes developed round the ovum. By Kowalevsky (No. 58) these bodies are regarded as true cells, and are believed to be formed by some of the cells of the original follicular epithelium making their way into the vitellus of the ovum and multiplying there. By Kupffer (No. 60), and Giard (No. 61), and Fol, they are also regarded as true cells but are believed to originate spontaneously in the vitellus. Finally by Semper they are believed not to be cells, but to be amoeboid protoplasmic bodies which are pressed out from the vitellus under the stimulus of the sea-water or otherwise. They do not according to this author naturally appear till the ovum is quite ripe, though they can be artificially produced at an earlier period by the action of reagents or sea-water. When produced in the natural course of things the vitellus undergoes a contraction. They are without any apparent function, and play no part in the embryonic development. Semper's results are very peculiar, but owing to the careful study which his paper displays they no doubt deserve attention. Further investigations are however very desirable. Kowalevsky from his researches on Pyrosoma (No. 59) adheres to his first opinion, though he abandons the view that these cells are connected with the formation of the test. In the passage of the egg through the oviduct the vacuolated follicle cells grow out into very peculiar long processes or villi. In Ascidia canina these processes become as long as the whole diameter of the vitellus (Kupffer, No. 60). In Amphioxus and the Craniata the ova are developed as in the Chætopoda, Gephyrea, etc., from specialized germinal cells of the peritoneal epithelium.