SECT. 12] GYCLOSES, PHOSPHORUS, SULPHUR 1247 



at the gastrula and 1230 at the pluteus stage. This might be regarded 

 as being due to a removal of other dry substances from the egg by 

 combustion, but such an explanation cannot be correct, for as 

 Ephrussi & Rapkine showed, the total dry soHd in echinoderm eggs 

 tends to rise slightly during development owing to the intake of ash 

 from the sea- water. And even if there was a decrease of dry weight it 

 could not, judging from the respiration data given in Section 4-2, 

 amount to more than 5 per cent, of the initial dry weight. Yet the 

 phosphorus increases by 62 per cent, of its initial value. 



There is, indeed, nothing against the view that phosphorus is ab- 

 sorbed from the sea-water by certain marine invertebrate eggs, for 

 we know what remarkable accumulations of other elements, such as 

 strontium, can occur in foraminifera. As will be shown, moreover, in 

 Section 13-4, inorganic constituents are absorbed from the sea- 

 water by many marine eggs. If now we ask how the absorption of 

 phosphorus takes place we come upon several points which merit 

 consideration. It is usually surmised that echinoderm eggs hatch so 

 early and the larvae swim about so vigorously owing to the need for 

 wide dispersal of the species in the plankton, but there may also be a 

 chemical reason for this, the egg not being supplied with all that it 

 needs for its proper development and being therefore under the 

 necessity of going to fetch it. An analogous instance among aquatic 

 vertebrates might be found in those teleostean eggs which begin to 

 rotate within their envelopes at a very early stage, setting up intra- 

 ovular currents and more readily absorbing the water which they 

 require. In order to gain some idea of the speed of movement of 

 echinoderm embryos Needham & Needham measured the time taken 

 for them to traverse a course on a micrometer scale, with the following 

 results : 



Assessing thus the average speed ^ at 1-5 metres per hour we may 

 consider an echinoderm gastrula passing for one hour through a tube 

 of sea-water of its own diameter. As this is 0-196 mm. such a tube 

 would contain 4-515 c.c. of sea-water, and in this there would be 

 1-56 X io~*mgm. of phosphorus, if we take the phosphorus-content 



^ See also Runnstrom. 



