198 H. von Mohl on the Investigation of Vegetable Tissue 3. The rhachis occurring in the vitellogene of certain Nema-toidea is never an apparent one in Meissner's sense, but always an actual one. 4. Meissner's micropyle in the eggs of Ascaris mystax does not exist. BischofF and Thompson have disputed its existence with perfect justice. 5. Whether the fecundation of the eggs is, or is not, effected by the penetration of the seminal corpuscles, remains unde-cided. At any rate, the observations published by Nelson and Meissner upon this point are insufficient to establish the pene-tration. 6. Meissner's theory of the conversion of the seminal cor-puscles into fat is destitute of any solid foundation, and can by no means be sustained. 7. The formation of fat-drops takes place in great proportion in the unfecundated eggs. 8. Schneider^s statement as to phsenomena of movement in the seminal corpuscles of certain Nematoidea is founded upon very accurate observations, which are confirmed not only by our own, but also by those of G. Wagener and N. Lieberkiihn. XX. — On the Investigation of Vegetable Tissue by the aid of Polarized Light. By H. von Mohl*. Polarized light offering a most sensitive means of discovering very slight differences, such as cannot be detected in any other way, between bodies which in every other respect behave exactly alike, the idea readily suggested itself of applying it in com-bination with the microscope to the examination of the struc-ture of organic bodies. In addition to the isolated observations of Biot, Brewster, and others, we possess more comprehensive treatises on the investigation of vegetable tissue by means of the polarizing microscope, by K. von Erlach, Ehrenberg, and Schacht f. Of these, Erlach occupied himself more with the physical explanation of the phsenomena which doubly-refracting substances exhibit on the polarizing microscope, than with ex-tended researches upon vegetable structures. The most general conclusion which Erlach drew from his re-searches was, that every organic substance of a certain thickness, * Botanische Zeitung, January 1, 1858. t Dr. Karl v. Erlach, MuUer's Archiv f. Anat. u. Phys. 1847, p. 313. Ehrenberg, Bericht Berlin, Akad. 1849, p. 56. Schacht, Pflanzenzelle, 1852, p. 429 ; Lehrbuch d. Anat. u. Phys. d. Gevvaclise, 1855, i. p. 428. Whether the works of Boeck shoukl be enumerated here, I know not, since I have not access to the originals, and the extract given by Han-nover (Miiller's Archiv, 1844) is too imperfect.
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