Lecture V. 39 



which must not be overlooked is that the response need not be 

 simultaneous with the stimulus which evokes it. The past 

 history of the cell with its countless stimuli determine to a large 

 extent its present behaviour. This so-called physiological memory 

 is probably bound up with the colloidal nature of protoplasm and 

 other cell-contents. 



Within a culture of Chlamydomonas we not infrequently come 

 across a large cell-wall containing four ovoid cells. These inner 

 cells are sooner or later seen to begin to move within the large 

 cell-wall and they finally escape as complete Chlamydomonads, 

 in all respects like the cell we have already studied. The origin 

 of these cells has been observed. A mature Chlamydomonad 

 loses its cilia, its nucleus divides into two and each of these two 

 into two more, so that there are now four nuclei in the cell. The 

 chloroplast also subdivides and finally the whole protoplasmic 

 mass, by two constrictions at right angles to each other, divides 

 into four small masses, each containing a nucleus and chloroplast. 

 A cell-wall is secreted round each mass and finally the proto- 

 plasmic beak and its cilia are formed, i When all is ready the 

 wall of the original cell is partially liquefied (possibly by an 

 enzyme secreted by the young cells) and the new individuals 

 emerge. 



Sometimes the protoplasmic contents of a cell break up into a 

 much larger number of small cells so that thirty-two or even 

 sixty-four cells may be produced. These sixty-four minute cells 

 seem essentially similar to the larger ones. When they emerge 

 they swim about for a longer or shorter period and then fuse in 

 pairs. The fusion begins at the anterior ends. Each of these 

 fusing cells is called a gamete. The mass formed of the two 

 fused cells is called a zygote, and forms a cell-wall round itself. 

 This cell-wall becomes thickened and a red pigment is developed 

 in the protoplasm within. After a longer or shorter period of 

 rest, this cell, the zygote, germinates by the protoplasm subdivid- 

 ing into four or eight individuals which develop walls and cilia 

 and are complete reproductions of the cells which gave rise to 

 the gametes. The wall of the zygote partially liquefies and the 

 cells escape. 



The former of these two methods of reproduction is described 

 as asexual, the latter, involving the fusion of two cells, sexual. 

 The reproductive cells formed in the asexual process are called 

 spores. As a general rule the fusing gametes come from different 

 cells, and in some species of Chlamydomonas it is noticed there 

 is a difference in size between the sexual cells which fuse together. 



