Multicellular Organisms 33 



The simplest type of multicellular animal 

 (Metazoa) is to be found among the Sponges. 1 

 Only a slight degree of co-ordinate action exists 

 between the cells forming the bodies of these 

 animals, and they thus represent a primitive 

 grade of organisms beyond which other Metazoa 

 have passed. (See Appendix at end of Chap. 

 VII.) 



Following the Sponges, in the ascending 

 scale of the animal kingdom, we come to the 



human ovum at 2 V mm. and an atom at nn^oVou of a millimetre, 

 and assuming that about fifty exist in each organic molecule 

 (proteid, etc.), the cube would contain at least 25,000,000,000,000 

 organic molecules. Again, the head of the spermatozoid, which 

 is all that is needed for the fecundation of an ovum, has a diameter 

 of about ^J<r mm. Imagine it to be a cube ; it would then contain 

 25,000,000,000 organic molecules. When the two are fused 

 together, as in fecundation, the ovum starts on its life with over 

 25,000,000,000,000 organic molecules. If we assume that one half 

 consists of water, then we may say that the fecundated ovum may 

 contain as many as about 12,000,000,000,000 organic molecules. 

 Clerk Maxwell's argument that there were too few organic mole- 

 cules in an ovum to account for the transmission of hereditary 

 peculiarities does not apparently hold good. Instead of the number 

 of organic molecules in the germinal vesicle of an ovum numbering 

 something like a million, the fecundated ovum probably contains 

 millions of millions. Thus the imagination can conceive of com- 

 plicated arrangements of these molecules suitable for the develop- 

 ment of all the parts of a highly complicated organism, and a 

 sufficient number to satisfy all the demands of a theory of heredity. 

 Such a thing as a structureless germ cannot exist. Each germ 

 must contain peculiarities of structure sufficient to account for 

 the evolution of the new being, and the germ must therefore be 

 considered as a material system. See Nature, September 26, 1901, 

 p. 547. 



1 Treatise on Zoology, edited by Sir Ray Lankester, Part II., 

 p. 2, by Prof. E. A. Minchin, F.R.S., on Sponges. 



C 



