Herbert John Webber

“ All of the intervening cells between the central cell and the spermatids being considered as potential spermatids, it becomes evident that we should expect blepharoplasts or their rudiments to be present. The fact brought out by Shaw and Belajeff that these bodies apparently appear de novo in each cell generation and then at the close of the division disappear in the cytoplasm, new blepharoplasts arising meanwhile to function in the next cell generation, seems to the writer to lend strong support to his claim of the independent nature of the blepharoplast. ”

“ In none of these cells is a cellulose wall laid down, there being nothing but a plasma membrane or hautschicht formed. The first prothallial cell is shaped like a plano-convex lens and arches out into the second prothallial cell. In the mature pollen grains it can not be determined whether the plasma membrane formed in cutting off this cell is attached at the sides to the plasma membrane of the tube cell, as must be the case if only two prothallial cells are formed, or to that of a third resorbed prothallial cell. ”

“ If the series of sections of a pair of spermatids is traced through till the upper section is reached the surface of the single band will be found to stretch entirely across it above the nucleus. To reach this location from the point where it was organized by the blepharoplast granules, the band has traveled almost the entire width of the cell. It can be observed in this stage to be in close proximity to the plasma membrane, but evidently not fused with it, as the writer was at one time inclined to believe. ”

“ The fact that a double plasma membrane delimiting the daughter cells is first formed in cell division, each cell having its own membrane, and that if a cellulose wall is formed at all it appears later between these membranes, indicates that the wall is of secondary importance, which is further supported by the fact that in many cell divisions, as in all the prothallial cells of Zainia and Ginkgo, no cellulose walls are formed, the cells being delimited only by the plasma membranes. ”

“ Structure and form of the mature spermatozoid The mature sperniatozoids, before they start to swim, differ but littie in appearance from the spermatids in the last stage described (fig. 42) . In all cases the two spermatozoids which result from the division of the same central cell remain attached together until active motion starts. When mature, and before separating, each spermatozoid is irregularly hemispherical, in shape (fig. 45) . The nucleus occupies a large portion of the cell, but is plainty surrounded on all sides by a layer of cytoplasm. ”

“ The body cell corresponds to the central cell of an antheridium; it increases in size more than double, and its nucleus is enlarged in the same proportion. Hereupon this central cell undergoes commonly a cross or oblique division, by means of which two generative cells are created. The stalk cell of the antheridium is divided apparently only under certain conditions. Then the stalk cell and the first prothallial cell lose their independence, and the liberated generative cell passes into the pollen tube. ”

“ The spindle fibers have bulged out on each side of the old mother-cell nucleus, reaching the cell wall on each side. They have contracted away from the daughter nuclei, and seem at their outer ends to be in the process of gradual transformation into the normal reticulum of the cytoplasm. No visible thickening occurs on the fibers where the new plasma membrane is to be formed, but a space free of granules and stainable matter clearly shows where the new plasma membrane delimiting the two cells will form. This is particularly well shown in a photomicrograph of the cell shown in figure 63. ”

“ One of the most typical cases of centrosomes in plants is that described by Swingle in 1897 as occurring in Stypocaulon (114) . Here a minute, deeph' staining, dumb-bell-shaped body occurs at the pole of the spindle in karyokinesis, which at the close of division divides into two. Both of these remain in close connection with the nuclear membrane, but travel in opposite directions until they come to lie at opposite points on the equator of the nucleus. ”

“ The plasma membrane of the pollen-tube cell is not so easily differentiated as that of the cells of the prothallial apparatus, but is usually clearly distinguishable, and would probably form an obstruction to any object like a spermatozoid entering it. Considering the structure and phenomena presented, the writer has been led to the conclusion that the description of the structure given in his preliminary paper is correct, and that Hirase and Fujii probably erred in figuring a mother cell in Ginkgo, inside of which the spermatozoids are differentiated. ”

“ Another feature of importance in the spermatozoid formation of Zamia is the metamorphosis of the entire spermatid cell into a spermatozoid. In the writer's preliminary papers it was pointed out that when the central cell divides to form the spermatids a plasma membrane is formed entirely across the cell, and that in the transformation of the two daughter cells or spermatids into the spermatozoids these cells are transformed directly into the spermatozoids without the formation of a new plasma membrane. ”

“ In quite a number of cases the blepharoplast could be plainly distinguished, and in one instance shown to the writer by Mr. Bessey the contents could be seen to be vacuolated or reticular almost as plainly as in the fixed and stained material. There can thus be no doubt that the main features brought out by fixing and staining in Ginkgo and Zamia are perfectly normal and not artifacts. The study of the living material of Ginkgo, which should be carried much farther, promises much valuable information, particularly in confirming the results obtained by fixing and staining. ”

“ If it is a centrosome, as claimed by some writers, does it function as a centrosome in the segmentation of the eggi It seems to have been established that in many animals the spermatozoid brings in the centrosome which forms the amphiaster for the first division, but this has not yet been proved in the case of any plant. That the blepharoplast or the cilia-forming organ enters the egg and remains in the cytoplasm in Zamla and Cycax is certain and not open to any question. ”

“ The prothallus grows in size proportionately more rapidly than the other organs, and this is accomplished largely at the expense of the nucellus, which gradually becomes thinner throughput, and is finally, at the time fecundation occurs, found to be compressed to a very thin membrane at the apex, and below on the sides and base has largely split up into very thin shreds, seldom being found as an unbroken membrane throughout. ”

“ The spindle does not occupy the entire nucleus, and at each side the protoplasin retains the reticular form similar to its structure before the spindle began to form. The most noteworthy variation in structure occurring in the cell at this stage of the division is in the blepharoplast, which has undergone a striking change since the last stage. ”

“ The plasma membrane (Ilaut-schichf) , from which the cellulose wall is apparently secreted, probably draws away from the worthless cellulose shell in the spermatozoid formation so that the entire cell, morphologically, is utilized. Nothing is lost in nature as a usual thing. How then could we expect the plasma membrane, which is apparently simply a modified form of active kinoplasm, to be thrown away ? ”

“ Before the pollen tube has increased in length very greatly it increases also noticeably in width, and by the time it has reached a length four or five times as great as the diameter of the pollen grain it has reached a diameter as great as that of the pollen grain (fig. 13) . The point where the tube bursts out of the pollen grain has also enlarged, the broken edges of the exine being bent outward. Mean-while the tube nucleus has assumed a round form, increasing some-what in size.
The protoplasm presents a beautiful foam structure, with large vacuoles here and there. ”

“ The reorganizing daughter nuclei, in the only section secured in this stage, are too densely stained to show their structure well; they appear simply to be densely granular. The spindle fibers show very plainly, but do not as yet show any thickenings in the center preparatory to the formation of a cell membrane. A most careful search has failed to reveal any suggestions of a centrosphere or centrosome at the apex of the spindle where one might be expected to occur. ”

“ It seems surprising, however, that no radiations extend out from the blepharoplast into the cytoplasm on other sides than toward the nucleus when the spindle fibers would appear from the figures to be so plainly visible. The centrosome usually forms the center of an aster, the rays of which extend out in all directions. Yet judging from BelajefFs figures there is no indication of such radiations in Marsilia. It would seem possible that in Marsilia the blepharoplast ma3 T be independent of the spindle, though occupying a position near the meeting point of the converging spindle fibers. ”