TEANSACTIONS OP SECTION D. 



715 



teen worked out only in the other, and are contained in records never likely to be 

 consulted in reference to the given problem. Thus, the ' contractile vacuole _' is an 

 oro-an that exists in both kingdoms: the question of protoplasmic vacuolation,_of 

 which this is a particular case, has only been really studied by the vegetable physio- 

 loo-ist; and yet in the minute plant-cells which possess this organ it is too small for 

 the study of the mechanism of its work : this study has been successfully carried 

 out by zooloo-ists on the larger Ciliata and Heliozoa, in ignorance, however, of the 

 known facts that explained the reason of its working. Thus the botanists had 

 foro-ed a key of the right pattern, but too big to open the locks m their own 

 domain, while fitting that of which the zoologists had described the wards and 

 tumblers. In this preliminary publication I shall only put the key into the lock, 

 reserving original observations for a complete paper. 



I. Distribution.— One or more contractile vacuoles occur in all naked plant 

 zoospores, with scarcely a recorded exception, whether mastigopod or myxopod, 

 whether Algal, Fungal (Saprolegnieffi, Peronosporese, Chytridiese), or Myxomycete. 

 They occur in all fresh-water Protozoa when in the active state, though not when 

 encysted ; they are, however, absent from those that live in the perivisceral fluid or 

 blood of living hosts from the Eadiolaria, and possibly other marine forms. 



II. Positio7i and Mechanism.— They are vacuoles in the protoplasm, usuaUy 

 peripheral; they contract to disappearance at regular intervals; in favourable cases 

 (A-ctinosphserium, many Ciliata) they are seen on contraction to discharge their 

 contents into the water. In a few cases they are known to be reservoirs filled 

 gradually by the almost continuous influx from plasmatic canals, which are invisible 

 from this very reason, only starting into sight normally during the momentary 

 pause when, owing to the contraction of the vacuole, their liquid contents have 

 time to accumulate and distend them. 



Now, if a specimen of sufficient size, say a Paramcecium, be placed under 

 certain unfavourable conditions (among which is deficiency of oxygen), we find 

 that the contractile vacuoles contract less perfectly and at longer intervals ; the 

 plasmatic canals become persistently visible and enlarged ; the whole animal be- 

 comes dropsical ; its thin cuticle bursts, and its protoplasm, no longer protected 

 by the ' Hautschicht ' and cuticle, and exposed directly to the water, dismtegrates 

 into ' diffluence.' 



Again, if a vegetable cell be wounded in water and its protoplasm passes put, 

 it rounds oS" and surrounds itself with a Hautschicht; then vacuoles appear mside ; 

 these enlarge enormously and finally burst ; the protoplasm so exposed without a 

 Hautschicht to the water disintegrates into diffluence, just like the Ciliate whose 

 contractile vacuole works inefficiently. This pathological vacuolation and diffluence 

 do not occur if to the water there be added a sufficiency of sugar, saltpetre, glyce- 

 rine, or other innocuous osmotic substance ; and when the protoplasm has the power 

 of excreting a complete cellulose wall, as in Vaucheria, the incipient vacuolation is 

 arrested on the formation of the wall. . 



The explanation of both series of facts is the same, as worked out by a series ot 

 botanists from Von Mohl to De Vries : — • i •+ 



Protoplasm contains in its interstices substances of high osmotic value ; its 

 outer layer at least, while freely per\-ious to water, is slightly if at all pervious to 

 these substances even in solution. Hence, when protoplasm is immersed in water, 

 cavities or vacuoles form in its substance containing solutions of these substances, 

 which continue to enlarge by attraction of water from without ; the enlargement 

 produces a tension which DeVries and others have by various methods determined 

 to be at least 3 atmospheres, and which may reach 15 atmospheres. Now, naked 

 protoplasm has very little toughness, it yields readily to the increasing tension and 

 to the expansion of the vacuole, and finally bursts and disintegrates. _ On the other 

 hand, cellulose and chitinous cyst-walls are sufficiently tough to resist; and equi- 

 librium is attained when, after a certain amount of stretching, the elasticity of the 

 wall balances the tension of the vacuoles due to osmosis. 



If, however, in the absence of a tough wall, the vacuole, instead of extending 

 indefinitely and bursting irregularly, (a) opens by a mmute pore, (b) contracts 

 regularly as it expels its contents, (c) closes up simultaneously with the completion 



