Examples of Pupils' Work 179 



ectoplasm. The endoplasm contains structural constituents 

 of the cell-body, such as the trichocysts and the macro 

 and micronucli, which lie just above the mouth. The 

 macronucleus is a relatively large structure. It has a 

 definite nuclear membrane which appears to be filled with 

 minutely granular contents. The micronucleus is a small 

 ovoid body, placed alongside of and close to the macro- 

 nucleus. 



Also two large contractile vacuoles occupying a con- 

 stant position in the deeper part of the cortical layer, one 

 near either end of the body. Each contracts alternately 

 with great regularity at intervals of about 10-20 seconds. 

 Just before contraction the vacuole appears as a large 

 clear space in the ectoplasm ; its walls come together and 

 its fluid contents are expelled to the exterior. The ensu- 

 ing pause, during which the vacuole is refilled with fluid, 

 is known as diastole. Immediately after the contrac- 

 tion a number of fine canals make their appearance in the 

 endoplasm, radiating from the spot where the vacuole 

 disappeared by closure. These canals reach for some 

 distance into the surrounding cell-body, but those of one 

 vacuole do not communicate with those of the other. The 

 canals become swollen with fluid at their inner ends and 

 slowly empty their contents into the vacuole, which reap- 

 pears, gradually filling with the fluid poured into it by 

 the different canals till it reaches its largest dimensions, 

 and then it suddenly contracts again. Towards the close 

 of the diastole the different canals, having emptied their 

 contents into the vacuole, become altogether indistinguish- 

 able. 



The substance of the endoplasm is loaded with a num- 

 ber of granules and particles, the products of assimilation 

 and metabolism. The most noticeable substances in the 

 endoplasm are the food vacuoles and the food they contain. 

 Food and water vacuoles are constantly re-appearing 

 and disappearing. The endoplasm is the seat of the 

 digestive, and to a great extent of the anabolic, activities of 

 the animal. 



d. Organization, etc. The Paramecium shows polarity, 

 that is, its parts are grouped symmetrically with respect 

 to an organic axis passing from pole to pole. Its nucleus, 



