Several days ago I mentioned the work of Wilhelm His, who modeled the neural tube after a flexible rubber tube. He noted that a rubber tube could be manipulated into forms that resembled the neural tube of the chick embryo, at various stages by pulling strings attached to certain places on the rubber tube. The strings represented forces that were applied to the actual neural tube by unknown agents. This did not sit well with the popular theory at the time – the biogenetic law of Ernst Haeckel: ontogeny recapitulates phylogeny. This theory posited that the embryo of an animal must develop through its ancestral stages before it assumes its proper form in its place on the phylogenetic tree. For example, human embryos must assume the forms ‘lower’ vertebrates in succession it assumes the human form. His’s proposed model annoyed Haeckel to no end, since the mechanical model ignored the genetic process. In fact Haeckel, intending great disparagement, called His’s theory, Gummi- schlauch Theone (rubber hose theory).
Flexibility of the neural tube *
Is the neural tube actually as flexible as a rubber tube balloon? Karl Ernst von Baer, noted embryologist of the early 19th century, introduced the idea that the embryo can be understood as constructed of layers or sheets. He argued that these sheets of cells were inherently flexible, enough to be bent or folded into larger tissues and organs. Von Baer’s theory was proposed 10 years before the idea that tissues layers were made up of cellular units (as put forth in Microscopical Researches into the Accordance in the Structure and Growth of Animals and Plants, published in 1839 by Matthias Schleiden). Baer proposed that there were three sheets which he named on the basis of their location, accordingly from the outside of the body inward, ecto- meso- and endoderm. Ectoderm was the sheet of tissue on the outside of the body (equivalent to the skin); endoderm was the innermost layer that would form such structures as the intestines, and the mesoderm was situation in between these two. The neural tube is an in-rolling of the outer ectodermal layer.
In later posts we discuss the formation of the neural tube.
* Lautin, A.L., Davis, J.I. Introduction to Neuroanatomy (in publication)