D.— ZOOLOGY. 67 



This is obviously more likely to take place where certain definitely digestive 

 cells have been segregated in the wall of the blastosphere, as in the ontology 

 of a metazoon. 



With the completion of a closed cellular envelope externally flagellate 

 and internally filled with jelly, we have now the possibility of any true 

 sponge (excluding the hexactinellids) and of any metazoon. We scarcely 

 note the mucilaginous exudation in free flagellates except when they 

 cling together, though we observe it in the fixed forms as the investing 

 jelly of the palmellar stage, or as the gelatinous ' houses' of choanoflagel- 

 lates and others. But, enclosed in a hilosphere, it has now the potenti- 

 alities of the intercellular jelly of the sponge parenchym, of the structureless 

 lamella of the hydroid or the jelly of the medusa disc, of the semi-fluid 

 which fills the segmentation-cavity in every larva and embryo. With 

 slight modification it has become in us the jelly of our cartilage and the 

 plasma of our blood. 



The late Dr. Strangeways and others, who have cultivated tissues in 

 vitro, have shown that living tissue, from almost any source except highly 

 differentiated epithelium, when placed in a nutrient medium, will proliferate 

 feathery cells into the medium until a network is formed of nucleate 

 masses of protoplasm stretching fine processes into the delicate threads 

 which join them. 23 That form of proliferation into a nutrient medium 

 must have been inherent in the flagellate ancestors of true sponges and 

 of metazoa. But we get a cobweb growth of similar form into sea-water 

 in the flagellate Dendromonas,'* 4 and in the hexactinellid sponges, as 

 described by Ijima. 25 We have no idea of the conditions which determine 

 coalescence or independence, 26 but the ' buffy coat ' of human blood, 

 which is a measure of the agglutination of the red corpuscles, may be 

 increased a hundredfold by some change in the constitution of the plasma, 

 the nature of which is still the subject of surmise. 27 In true sponges, 

 coelenterates, and trochospheres, we find the surface toward the water 

 preserving the firm outline of an epithelium, while the surface toward 

 the internal nutrient medium, as in laboratory preparations, proliferates 

 into the segmentation-cavity feathering cells — named by the Hertwigs 

 mesenchyme — to form the tissues of the organism. This is well shown in 

 Baitsell's drawings of the segmentation-cavity in the chick embryo. 28 



The exudation from the cell-surface, which our cartilage and ganglion 

 cells have inherited from the flagellates, is the foundation of metazoan 

 physiology and of metazoan morphology. 



We must still ask, with what advantage did sponges and Metazoa 

 arise in the Flagellate Sea of the early Pre-Cambrian ? Among the photo- 

 synthetic monads we know that there were some which learnt the canni- 



23 E.g. T. S. P. Strangeways, 1924 : ' Tissue Culture in Relation to Growth and 

 Differentiation. ' Cambridge, Heffer. 



21 Pascher, 1914 : Siissu-asserflora, Jena, Fischer ; i. p. 95. 



"' 'Studies on the Hexactinellida,' Tokyo : Jour. Sci. Coll. xv, pi. 5, and passim. 



26 J. Gray (1926: Brit. Journ. Exp. Biol, iii, p. 167, quoting Galtsofi, 1925) has 

 results allowing the deduction that coherence was only possible while the sea 

 retained a certain acidity or after it had reached a certain salinity. 



27 C. Lovatt Evans, I.e. p. 8. 2 » G. A. Baitsell: Q.J. M.S. 1925, lxix, p. 571. 



F2 



