60 SECTIONAL ADDRESSES. 



These spheres we may perhaps call ' hilospheres,' 19 because of the hiluin or 

 scar left at the point of ultimate closure, leaving the name ' blastospheres ' 

 for spheres produced by the secondary process of holoblastic segmentation 

 in three Cartesian planes, which has been adopted by the metazoan zygote. 

 This, like so much of the phenomena of embryology, must be regarded 

 not as a repetition of history, but as a direct method of obtaining a result 

 previously reached by a longer process. It is interesting that the zygote, 

 both of the Calcaronean sponges and of the Ctenophora, clings to primitive 

 flagellate longitudinal division up to the 8-cell stage, producing a crown 

 or ring of cells strongly reminiscent of the 8-celled crown of the flagellate 

 Stephanosphaera or of the 16-celled Cyclonexis or Gonium. 



When a hilosphere has been closed, the mucilaginous secretion on the 

 inner surface of the cells which bound it can no longer be washed away, 

 and the interior cavity becomes filled with jelly. In sponges, the com- 

 paratively uniform diameter of the adult flagellate chambers in a given 

 sponge indicates that they cease to grow when the hydraulic pressure 

 of the water within them is equal to the static pressure in the interior 

 of a bubble, which is of the diameter of the chamber and has the surface 

 tension of its surface. That is, a collar-cell only divides longitudinally 

 when it is pressed upon by its neighbours on either side, dividing against 

 this lateral pressure. It is possible that a similar response determines 

 the diameter of a closed hilosphere ; inside it there is not only the pressure 

 at which the jelly is secreted, but unless the cellular envelope be 

 impervious there will also be osmotic pressure due to the jelly not being 

 isotonic with the external sea-water. In the plasma of the frog's blood the 

 osmotic pressure is about 3 cm. of water for each 1 per cent, of proteins 20 ; 

 in the flagellate chambers of sponges I have published a calculation de- 

 ducing the surface-tension of - 35 C.Gr.S. units. 21 This, in a hilosphere or 

 blastosphere 60|i. in diameter, would be balanced by an internal pressure 

 of 1*2 mm. of water. Therefore a percentage of '04 per cent, protein 

 in the central cavity would produce equilibrium, the cells of the 

 spherical wall would neither be pressed against each other nor dragged 

 apart, and on the above hypothesis the growth of the hilosphere 

 would cease. 



It is interesting to note that, apart from any such hypothesis, if at any 

 stage the cells of a blastosphere or hilosphere proceed to feed on the fluid 

 in the segmentation-cavity and to withdraw proteins, carbohydrates, 

 or salts, the osmotic pressure will be lowered. If the central fluid be made 

 hypotonic as compared with sea-water 22 then the internal osmotic pressure 

 will be negative, and if the wall of the blastosphere be permeable to water 

 it will be invaginated. If from the central cavity the cells of a blasto- 

 sphere abstract soluble substances the blastosphere must be invaginated. 



ln This word will not bear philological analysis ; but it is convenient in length and 

 sound. 



20 C. Lovatt Evans, 1925 : ' Recent Advances in Physiology,' p. 148. London, 

 Churchill. [See also Adair, 1925, P.R.S., B. 98.] 



21 Q.J.M.S. 1923, lxvii, p. 300. 



22 At a recent meeting of the Challenger Society observations were quoted 

 showing that the urine and pseudo-cartilage of the sun-fish is each lighter than sea- 

 water. I regret that at the moment of going to press the references are not 

 to hand. 



