1882.J 



MICEOSCOPICAL JOUKNAL. 



29 



limiting membrane, the cell-wall, as 

 an essential element in the ultimate 

 life-unit. Even in cell-multiplication 

 by the ordinary process of division, 

 the wall takes no part, for within the 

 parent cell it is the protoplasm that di- 

 vides into two or more parts, begin- 

 ning with the nucleus. A constriction 

 forms in a certain plane, and new 

 cell-walls are secreted by the two 

 masses of living matter. As the cells 

 increase in size the original mem- 

 brane must give way or disappear. 

 But it may still be said that the phy- 

 siological unit of life is the cell; for, 

 although naked bioplasm may live, 

 assimilate food and grow, no differ- 

 entiation of parts can result until 

 there is some product, a secretion, a 

 cell-wall or limiting membrane, to 

 give form and structure to the primal 

 elements of growth. 



The living jelly, so readily studied 

 in the amoeba, seems to be identical 

 with the protoplasm of every animal 

 and plant from the highest and most 

 complex down to the lowest and sim- 

 plest. 



Let us, therefore, examine this pro- 

 toplasm more carefully. As the amoe- 

 ba moves, one portion of the body is 

 projected forward, and the less dense 

 protoplasm within begins to flow in 

 the direction of the projection, like 

 so much water, carrying with it 'the 

 spherical granules which are usually 

 abundant in the body. 



The amoeba has no mouth, but 

 when a digestible morsel is found the 

 body simply flows around and en- 

 velopes it, and the process of assimi- 

 lation immediately begins ; the indi- 

 gestible portions are allowed to es- 

 cape from any part of the surface of 

 the body. Hence, it appears that pro- 

 toplasm has the power of assimilat- 

 ing solid food and converting it into 

 living matter, which is the process of 

 growth. When the amoeba attains a 

 certain size, a constriction forms 

 across the body, or gradually deepens 

 until the animal becomes divided in- 

 to two parts, which finally separate 

 and move away as independent indi- 

 viduals. 



This simple process of propagationis 

 typical of what takes place throughout 

 the living world. Among the simplest 

 forms of life it serves for the multi- 

 plication of individuals, but as we as- 

 cend the scale the prpcess of repro- 

 duction becomes more complex, and 

 division of the constituent cells be- 

 comes a process of growth rather 

 than of reproduction. We may, in- 

 deed, regard the human body as an 

 assemblage of units, each of which 

 multiplies by division, like the simple 

 amoeba, and thus contributes to the 

 repair of waste in the tissues. The 

 process begins in the germinal cell, 

 and by its continuance the complex 

 organs of the body are evolved, ac- 

 cording to some inscrutable law. The 

 reproductive process of the amoeba, 

 therefore, typifies the growth of high- 

 er organisms ; for the first considera- 

 ble advance in structural evolution is 

 in the production of a more complex 

 organism by the division of cells, the 

 progeny of which, instead of separat- 

 ing from the parent-cells as new indi- 

 viduals, remain as integral and inter- 

 dependent parts of one organism, 

 each cell, or group of cells, having 

 specific functions in the economy of 

 the animal or plant. 



Among the green, confervoid algce 

 of ponds and ditches are found many 

 plants which consist of a series of 

 cells attached end to end, forming 

 filaments. These plants are termed 

 multicellular, to distinguish them 

 from the unicellular species, and they 

 are classed higher in the scale of or- 

 ganization. But complexity of struc- 

 ture, as thus manifested, is not, so far 

 as my judgement permits me to ob- 

 serve, an indication of a higher stage 

 of cell-life ; for each cell of the fila- 

 ments is complete and independent 

 of all the others. There is no physiolo- 

 gical bond connecting them, as in the 

 higher plants, but each one carries on 

 an independent existence, and is not 

 killed if its fellows are destroyed. 

 Between the filamentous plants and 

 the strictly unicellular forms which 

 consist of spherical, green cells, living 



