CYTOPLASM The cell consists of two parts, nucleus and cytoplasm. The former contains the chromosomes, which are the physical basis of heredity, and which probably bear most of the factors of inheri tance. Outside the nucleus is the body of the cell, formed of protoplasm. Floating in this semi-viscid protoplasm are always found many granular or rod-shaped bodies. Modern studies have shown that these bodies are of two types, which can be differen tiated from each other by special staining methods. The more numerous granules are known as mitochondria. These were first seen and imperfectly described by the German cytologist Flem ming, and were afterwards studied by Altmann (188o) . Altmann was wrong in the interpretations he placed on these granules in his "Elementarorganismen," but was correct in his description of them. Subsequently a great deal of work was carried out on the mitochondria by Benda, Meves, Duesberg, Guilliermond, Regaud, Cowdry, Gatenby, Ludford, Bowen and many others. It has been claimed from time to time by various workers that these granules are artefacts produced by the technique used, but the mito chondria can be seen and studied intro vitam.
In 1898, the Italian neurologist, Golgi, described some peculiar argentophile bodies in nerve cells, which form the Golgi appara tus, so called from its discoverer. As with the mitochondria, certain observers have claimed that the inner apparatus of Golgi is merely an artefact produced by the silver impregnation bath. Happily these structures can be seen and studied in many cells, intra vitam, as for instance in the spermatocytes of various Mol lusca, in which both Golgi apparatus and mitochondria are readily visible, and the presence of these bodies in all cells at some period of their existence is now generally accepted. The Golgi apparatus of nerve cells is a remarkable network (fig. 8) formed of ap parently anastomosing threads. In England and America in recent years various cytologists (Gatenby, Ludford, Brambell, King, Bowen, E. V. Cowdry, Hyman, etc.) have followed out the behaviour of both Golgi bodies or elements as they are now called, and mitochondria, during the gametogenesis and fertilization of many organisms, and the technical methods for the study of these cell bodies have been made surer. The morphology of the Golgi apparatus in various cells is depicted in figs. 1-2o. The classic network described by Golgi is shown in fig. 8. The apparatus here takes the form of branching rods, or canals filled or formed of a conspicuous argentophile or osmiophile substance. Now this type of Golgi apparatus has been shown to be highly specialized. In the more primitive type, as shown in figs. 3, 4, 6, 7, the appara tus in figs. 6 and 7 is formed of argentophile or osmiophile granules, platelets or rods, associated with a less densely staining substance. This formation holds good for all animals, but is often found only in embryonic and non-differentiated cells. This type of Golgi apparatus is characteristic of invertebrate cells. In the ordinary mammalian body cells the Golgi apparatus is usually of the appearance depicted in figs. 3 and 4, being juxta-nuclear and eccentric. The morphology of the mitochondria in various cells is shown in figs. 1 to 20.
The mitochondria are, as often as not, small spherical granules which lie scattered through the cytoplasm in no special formation, though in embryonic cells and in nearly all germ cells, at some period the mitochondria lie in a discrete mass near the Golgi bodies, as in figs. 9 and 13. In many cells the mitochondria are elongate or rod-like. During the life of the cell, the Golgi bodies and mitochondria are carried around in those cells which exhibit protoplasmic streaming, but in all cells the Golgi bodies and mito chondria at some period have the power of binary or multiple fission, and of growth.
During cell division the Golgi bodies either lie quiescent in the cytoplasm and are roughly divided between the two daughter cells as depicted in fig. 6, or they are attracted to the asters and hap hazardly sorted into two lots as shown in fig. 7. It is important to notice that neither the mitochondria nor the Golgi bodies are divided between the daughter cells in the remarkable manner in which the chromosomes are halved. This fact shows that even if the cytoplasmic inclusions do carry hereditary factors, these are of a much lower order of importance than those borne by the chromosomes. Probably all modern cytologists are agreed that the f unction of the mitochondria and Golgi bodies is trophic ; i.e., concerned with the growth and elaboration of cell products under the direct stimulus of the chromosomes within the nucleus. The hypothesis of Meves that these bodies bear the hereditary factors of the cytoplasm has not been sustained by recent ob servations.
In gametogenesis the mitochondria and Golgi bodies act very definitely in all animals, and it is in this field, and in the field concerned with cell secretion, that modern investigations have been most successful. In figs. 9-12 are stages in the formation of yolk in a mollusc egg. The small eggs (oogonia) have a dis crete juxta-nuclear eccentric Golgi apparatus surrounded by the granular mitochondria. In the growth of the egg the latter in crease in number and spread out in the cytoplasm; the vitello genesis or yolk formation concerns the Golgi bodies almost ex clusively. The original Golgi apparatus breaks up into a number of parts which wander out into the cytoplasm, growing and divid ing until a large number are formed. These smaller parts undergo the changes depicted in fig. 12, a–d, forming vacuoles which be come filled with dense fatty materials. At an early stage after their formation the contents of the vacuoles stain bright red in the intra vital dye, Neutral Red. In some animals, nucleolar extrusions, or pieces given off from the nucleolus, pass through the nuclear membrane into the cytoplasm where they may form yolk (NE). The ripe egg thus contains yolk granules which may be produced either from nucleoli, Golgi bodies, or from mito chondria, and also unchanged Golgi granules and mitochondria. During segmentation of the egg the granules of all types are distributed between the segments, and ultimately pass into the cells of all organs. .
In spermatogenesis the behaviour of the Golgi bodies and mitochondria is also definite, even more so than in oogenesis (formation of the egg) . In figs. 13-17 are stages of the forma tion of a spermatozoon from the mother cell (spermatogonium, fig. 13) . The function of the Golgi apparatus is to form the perforatorium or acrosome of the sperm. In figs. 15 and 16 the Golgi body is seen to pass up the cell, become fixed on the nucleus and secrete a bead—the acrosome. The mitochondria form the middle piece of the sperm, as shown in figs. 16 and 17. The function of the mitochondria appears to be to store energy to be expended by the sperm flagellum (F) in its movements. The remains of the cytoplasm, a few mitochondria and the Golgi bodies are stripped off the ripening sperm (s).
In fertilization usually the whole sperm enters the egg, but only in a few cases is there satisfactory evidence that the mito chondria persist. In nearly all cases the tail dies, the nucleus (N) alone living to form the male pronucleus, being the vehicle of the father's hereditary factors. In gland secretion the Golgi bodies produce droplets or secretion granules in much the same way as is depicted in figs. 9-12 for the egg.
Mitochondria are well known in plants (fig. 2), and certainly identified. Two other categories of cell inclusions as well as canals probably exist (Bowen), and it seems certain that plants will be found to have cell inclusions much like those of animals. Guillier mond and Mangenot (fig. 5) have described Golgi bodies in plants. It is not yet certain whether these so-called Golgi bodies are not merely canaliculi, and whether they are homologues of the real Golgi substance in animals.