30 ORGANIZATION AND DEVELOPMENT OP ANIMALS IN GENEEAL. 



the so-called amoeboid motion ; it sends out processes, draws them 

 in again, and is able by such means to change its position. This 

 capacity of change of form is especially possessed by young undif- 

 ferentiated cells, which have not developed an outer membrane. 

 Such cells in their later growth usually develop a cell membrane., 

 which accordingly is not, as was formerly supposed, a necessary 

 constituent of the cell, but is merely an indication that the cell 

 has undergone a certain amount of differentiation from its early 

 indifferent condition. 



It has been already pointed out that the fundamental properties 

 which distinguish the life of organisms manifest themselves also 

 in the life of their constituent cells. According to our present 

 knowledge, cells always originate from pre-existing cells ; a process of 

 free cell formation, as conceived by Schwann and Schleiden, indicated 

 by the precedent origin of nuclei in a formative organic material, 

 has never been proved. 



Such a process may, however, take place when the formative 

 matter is the plasma of a cell, or of several cells fused together 

 (plasmoclium). In such cases we have a process of free cell forma- 

 tion (e.g., spore formation in Myxomycetes) which certainly is not 

 clearly marked off from a process of new formation within the mother 

 cell, and is to be looked upon as a modification of the so-called 

 endogenous cell formation. This leads us to a consideration of the 

 very widely distributed method of cell increase by division. When 

 the cell has reached a certain size by the absorption and assimilation 

 of nutrient matter, the protoplasm separates itself into two nearly 

 equal portions, this process being usually preceded by the division of 

 the nucleus. Each portion receives half of the original nucleus. 



During its division the nucleus undergoes, as has been recently 

 shown in many instances, peculiar differentiations and changes (fig. 

 18). It becomes spindle-shaped ; its contents take on the form of 

 longitudinally arranged strise, running from pole to pole of the 

 spindle ; the centre of each of the striae becomes thickened, giving 

 rise to a cross equatorial zone of granular matter, the nuclear plate 

 (thickened zone). The central thickenings constituting the nuclear 

 plate divide. Each half travels towards the poles of the spindle, 

 and becomes there enclosed in a clear fluid mass, which appears in 

 the protoplasm. From these two structures the new nuclei are 

 formed at the poles of the now dumb-bell shaped nuclear spindle, the 

 $true of which vanish during the constriction of the protoplasm, 

 which has already commenced and quickly progresses. The division 



