Coccospheres and the Origin of Coccolitlis. 



313 



(fig. 18). It is presumably to be inferred that it is finally extruded 

 between these latter, and takes up its position in the shell of the 

 sphere. 



The internal coccolith gives the same reactions as the outer ones. 

 When a coccosphere is acted on by acids, the internal coccolith, if pre- 

 sent, is the last (as we might expect) to dissolve (fig. 14), and in its 

 solution its radial striae are the last parts of it to disappear. Under 

 treatment with picric-nigrosine, the behaviour of a coccosphere con- 

 taining an internal coccolith is characteristic. First the external 

 coccoliths darken, and their striae and other details stand out with 

 great clearness ; as solution proceeds, a very dark coloration covers all 

 the outer coccoliths, and when this clears away, their solution is all 

 but complete. The internal coccolith then goes through the same 

 phases as the outer ones ; its striae become clearer, it darkens, and, as 

 it goes into solution, the whole cavity of the sphere becomes filled 

 with a dark green coloration. The appearance of this coloration is 

 sometimes very sudden ; it disappears with less rapidity. When it is 

 gone, the scanty protoplasm, nucleus, gelatinous and slimy envelopes 

 stained with the nigrosine are all that are left of the coccosphere. 



As a general rule, only one internal coccolith can be made out* in 

 each coccosphere ; sometimes, however, one mature coccolith and one 

 in a very early stage are found. In the coccosphere (fig. 16), which 

 contained the central trilobed body alluded to above, so far as I could 

 ascertain a ring-shaped coccolith was in contact with each lobe. But 

 this observation is open to doubt, as the coccosphere was mounted in 

 water, so that the internal coccoliths were only indistinctly seen, and 

 unfortunately the solution of both internal and external coccoliths 

 took place while they were not under observation. 



In the case of internal coccoliths which have almost reached maturity, 

 it is generally possible to perceive that they are somewhat larger than 

 any of the coccoliths already in position on the sphere. It would 

 appear from this that the coccoliths formed by a coccosphere in its 

 earlier stages are smaller than those developed later in its life history. 

 Indeed, measurements of coccoliths and coccospheres almost necessitate 

 this conclusion. Thus coccoliths are often found with their longer 

 diameter equal to 0-018 mm., while the internal measurements of some 

 of the smaller coccospheres could not accommodate a coccolith of these 

 dimensions. Again, it is found that the coccoliths with a simple slit- 

 like perforation in their central body are, as a rule, smaller than those 

 with D-shaped perforations ; so that we may with some probability 

 assume that the single perforation is the more primitive condition, 

 and that coccoliths with it only are formed in the coccosphere during 

 its earlier stages. The history of the development of the coccolith 

 also points in this direction. 



It appears that the extrusion of a coccolith to the surface must 



