The Wonders of Microscopy 307 



the microcosm of the cell. Inside each of man's cells there are 

 about two dozen chromosomes, and one of the authorities on cell- 

 lore speaks of each chromosome having the corporate individuality 

 of a regiment, the really indivisible living units being the beads or 

 microsomes which correspond to the men ! And to this must of 

 course be added the fact that we have many millions of these cells 

 in our body. Indeed, we are fearfully and wonderfully made! 



The Beginning of the Individual 



Every many-celled creature, which reproduces in the ordinary 

 way, starts on the journey of life as a single cell the fertilised 

 ovum. As we have made clear in a previous article, the usually 

 microscopic fertilised egg-cell contains, in some way that we 

 cannot picture, the initiatives or "factors" for the hereditary char- 

 acters of the living creature in question. But the microscope has 

 begun to reveal the little world within the egg-cell, and it has been 

 found possible to map out the way in which the factors for certain 

 characters are disposed in the chromosomes. Thus in the case of 

 the egg of the fruit-fly called Drosophila, it is possible to say that 

 the hereditary or germinal factor for, say, red eye or grey wing, 

 lies at such and such a level in one of the four chromosomes. It 

 would be difficult to find a wonder of microscopy greater than this. 



Yet this is but an instance of what goes on at a level of visi- 

 bility which only the microscope can reach. We know much in 

 regard to the permutations and combinations which take place 

 when the germ-cell is ripening shufflings of the hereditary cards 

 which throw some light on the origin of new departures. We 

 know something of the manner in which the paternal and maternal 

 hereditary contributions behave in relation to one another when 

 and after fertilisation takes place. We know much in regard to the 

 sequence of events in individual development, wherein the ob- 

 viously complex emerges from the apparently simple, and the 

 implicit inheritance becomes an explicit individual. In the seven- 

 teenth century, William Harvey, the discoverer of the circulation 



