144 DESIGN IN NATURE 



PLATE LX (continued) 



wall of the daughter-cell has absorbed water and burst ; the protoplasmic body having forced itself through the fissure and assumed 

 a rounded contour (after Sachs). 



Fig. 2.— a. Transverse section through a young internode of the stem of Tradescantia alUfiora. a, Vascular bundles; h, 

 large cells forming the parenchyma of the fundamental tissue (after De Bary). 



B. Transverse section of the flower-scape of Allium Schcenop-asum , x about 30. a, Epidermis ; 6, chlorophyll cells; c, colourless 

 parenchyma of the cortex ; d, parenchyma of pith ; e e', vascular bundles ; /, ring of sclerenchyma (after Sachs). 



Fig. 3._A. Pinus sylvestris. Radical longitudinal section through the wood of a vigorous branch, a, Cambial wood-cell ; 6, 

 older wood-cells ; c, c', c'", bordered pits of the wood-cells in order of age ; d, d, large pits where the cells of the medullary rays are 

 in contact with the wood-cells. , , i ,, j 



B. Tangential longitudinal section through the wood of Ailanthus glandulosa. a, Vessels ; 6, medullary rays cut across; c, wood 



^ ''^a Ipirogyra longata. Several cells of two filaments about to conjugate (a, h). Spiral chlorophyll bands (c) with starch grains 

 [d), circularly arranged, and small drops of oil. The nucleus (e) of each cell is surrounded by protoplasm, from which threads go to 



the cell wall. x i • i. 



D. Cells engaged in conjugation, a, Protoplasm of one cell passing into another cell. At 6, two protoplasmic masses have 

 already united. At c, the young zygotes are provided with a wall (after Sachs). 



While the German histologists maintain that everything proceeds from the cell, they differ as to their definition 

 of a cell. 



Max Schultze avers that a cell wall or envelope is not necessary to the existence of the cell, and Briicke 

 declares that a nucleus does not necessarily form a part of it. If, however, we ehminate the cell wall and the 

 nucleus, all that remains of the cell is the matter contained between the two, namely, the cell contents or proto- 

 plasm, and this is now regarded by Professor Huxley and others as the germinal matter, or physical basis of life. 



The history of the cell is instructive. 



Schleiden and Schwann (1839), the original promoters of the cell theory, were of opinion that cells were produced 

 from an amorphous mass (cyto-blastema), contained either within or between pre-existent cells. In this mass 

 round, structureless, or minutely granular corpuscles appear. 



Around these corpuscles, which constitute the nucleus, a cell wall is formed by molecular disposition, and between 

 the cell wall and its nucleus fluid is stored up. By this process a nucleated cell is produced. From a mass of such 

 cells, according to Schleiden and Schwann, all the textures may be developed. In their opinion " the cause of 

 nutrition and growth resides, not in the organism as a whole, but in the separate elementary parts — the cells." 



Professor John Goodsir (1845), to whose writings I refer further on, thus expresses himself : " As the entire 

 organism is formed at first, not by simultaneous formation of its parts, but by the successive development of these 

 from one centre (the germinal spot of the ovum), so the various parts arise each from its own centre, this being 

 the original source of all the centres with which the part is ultimately suppUed. From this it follows, not only 

 that the entire organism, as has been stated by the authors of the cellular theory, consists of simple or developed 

 cells, each having a pecuhar independent vitality, but that there is in addition a division of the whole into depart- 

 ments each containing a certain number of simple or developed cells, all of which hold certain relations to one central 

 or capital cell, around which they are grouped. It would appear that from this central cell all the other cells of 

 its department derive their origin. It is the mother of all those within its own territory." 



Goodsir attached much importance to the nuclei of the cells : these formed his " centres of nutrition." In 

 this matter he proved himself an advanced pioneer. The nucleus is now known to be the seat of the most extra- 

 ordinary and important changes in reproduction : it is also beUeved to be the centre of inheritance. " The 

 identification of the cell-nucleus as the vehicle of inheritance, made independently and almost simultaneously in 

 1884-85 by Oscar Hertwig, Strasburger, KoUiker, and Weismann, must be recognised as the first definite advance 

 towards the internal problems of inheritance through the cell-theory." 



Similar in many respects to Goodsir's theory of " centres of nutrition " is Dr. Beale's theory of germinal matter. 



Professor Huxley (1853) referred the origin of cells to vacuoles occurring in a homogeneous plasma. These 

 vacuoles contain the cell wall, nucleus, and contents. The cell wall he termed the periplast, the nucleus the 

 endoplast. He attaches no importance to the latter, in which respect he entirely differs from Goodsir. He says, 

 in fact, that not unfrequently the nucleus has disappeared before the really essential changes in the cell begin. 

 Huxley attaches especial importance to the periplast or cell wall and the matter contained within it. In these, 

 according to him, all the really essential, vital, physical, and chemical changes occur. It would be easy to write 

 volumes on the cell theory, but enough has been said to show that authors are not agreed either as to what 

 constitutes a cell, or as to the more important parts of it. Some think that the essential part is the cell wall or 

 envelope, others the nucleus, others the substance contained between the nucleus and envelope : some ao-ain are 

 of opinion that neither cell wall nor nucleus is necessary, and that the contained matter or protoplasm is the only 

 essential element. 



