How is this human body in all its complexity developed from the microscopic germ-cells? There has been a vast deal of ink spilled in striving to solve this mystery, but we come out empty by the same door wherein we went. The early Preformationists guessed that the ovum contains an embryo fully formed in miniature, and development is a mere unfolding of what had already existed. The biologists of to-day mention the Preformationists with superior scorn, and then present Preformationism under other names. Weismann's theory is the most fashionable at present.
In a paper read at the Darwinian Memorial Congress in 1909, Weismann said: "With others I regard the minimal amount of substance which is contained within the nucleus of the germ-cells in the form of rods, bands, or granules, as the _germ-substance_, or _germ-plasm_, and I call the individual granules[37] _ids_. There is always a multiplicity of such _ids_ present in the nucleus, either occurring individually or united in the forms of rods and bands (chromosomes). Each _id_ contains the primary constituents of the _whole_ individual, so that several _ids_ are concerned in the development of a new individual." Actually there are such things as chromosomes, and when these are stained and are under the highest power of the microscope they appear to be granular. These granules Weismann calls _ids_. Beyond the fact that there are such granules, all else is sheer guessing.
[37] _Id_ is a word derived from Nägeli's term idioplasm, which means the chromosome granule.
He says further: "In every complex structure thousands of primary constituents must go to make up a single _id_; these I call _determinants_, and I mean by this name very small individual particles, far beyond the limit of microscopic visibility, vital units, which feed, grow, and multiply by division. These determinants control the parts of the developing embryo,--in what manner need not here concern us."
There is some truth here. The _id_ is made up of molecules and atoms, ions and electrons, and in some manner, of course, these have to do with the development of the embryo; but as to the manner we have not the slightest knowledge, and just this knowledge is what we need to make the theory anything more dignified than a child's game at guessing. There is a structural differentiation in the unsegmented ovum, with all the embryonal axes foreshadowed in it, but this tells us nothing more than that the egg contains the man in germ.