I POLYSTOMELLA 23 



i.e. division not into two but into a greater number of new individuals. 

 The nuclei become all broken down into chromidia and the protoplasm 

 streams out into the pseudopodia forming a kind of halo round the now 

 empty shell. New round nuclei, probably formed from chromidia, now 

 make their appearance and the protoplasm divides up into rounded 

 masses each containing a nucleus and chromidia (Fig. 8, 8). These secrete 

 over their surface a thin spherical shell and if measured they will be 

 found to be from 60 jx to 100 /a in diameter. Each of these is a young 

 megalosphere. It creeps away by means of its pseudopodia. As it 

 increases in size the protoplasm bulges out and forms the character- 

 istic horn-shaped piece that fills the second chamber (Fig. 8, 9). As 

 growth continues other pieces are added on by a similar process and the 

 spiral form of the complete megalospheric individual is built up chamber 

 by chamber. During the process of growth the nucleus — the principal 

 nucleus — migrates onwards passing from chamber to chamber so as to 

 retain throughout its position about the middle of the whole mass of 

 protoplasm. 



As the megalospheric individual approaches its reproductive period 

 the principal nucleus disintegrates entirely into chromidia and then the 

 nuclear material concentrates secondarily to form very numerous small 

 round nuclei. The protoplasm becomes segregated round each of these 

 to form a small cell and this undergoes fission twice in succession so that 

 each original cell is represented by four. A curious quivering movement 

 may be seen in the interior of the shell and presently the contents issue 

 forth as a swarm of minute rounded bodies (Fig. 8, 2a) each provided 

 with a nucleus and possessing at one end a pair of fine protoplasmic 

 threads or flagella by the lashing movements of which the cell swims 

 actively through the water. Each of these flagellate cells simply dies 

 after a short time unless it happens to come across a similar cell (Fig. 8, 26) 

 derived from the breaking up of another megalospheric individual. If 

 this happens the two cells come into contact by their non-flagellate ends 

 (Fig. 8, 3 and 4), the flagella disappear and the two cells gradually undergo 

 complete fusion, not merely the cytoplasm but also the two nuclei becoming 

 indistinguishably fused together (Fig. 8, 5). The single rounded cell so 

 produced is found by measurement to be about 10 /i in diameter. It is 

 a young microsphere and gradually grows into a typical microspheric 

 individual. 



In this reproduction by the megalospheric individuals we have an 

 example of a sexual process — of what is known technically as syngamy. 

 This process consists essentially of the fusion together, and more especially 

 the fusion of the nuclei, of two cells termed gametes to form a single cell- 



