The German Genius
more polemical, an attempt to establish a general principle underlying all cell development, in plants as well as in animals. As he said in the foreword: “The aim of the present treatise is to establish the intimate connection between the two kingdoms of the organic world by demonstrating the identity of the laws governing the development of the elementary subunits of animals and plants. The main outcome of the investigation is that a common principle underlies the development of all the individual elementary subunits of all organisms, much as the same laws govern the formation of crystals despite their differences in shape.” The reference to crystals was of course much the same as what Schleiden had said and was, again, wrong. This error—an important one—was compounded, in the eyes of many, because Schwann hardly referred in his own work to others who had made contributions in the same field. Purkyne reviewed Schwann’s book and disputed Schwann’s claim for priority. 38
Whoever was first, Schwann’s book provoked more research. One profitable line of enquiry was followed by Franz Unger (1800–70), professor of plant anatomy and physiology at Vienna (who numbered Gregor Mendel among his pupils). 39 Unger collaborated with Professor Andreas von Ettingshausen (1796–1878), a physicist in Vienna, who had an excellent Plössl miscroscope, and they observed cell behavior that would eventually lead to the recognition of the importance of cell division. This was also a preoccupation of Carl Nägeli (1817–91) at the University of Zurich. 40 Nägeli thought that his early studies of cell division, published in 1844 and 1846, showed two types of cell formation: free cell formation and the division of pre-existing cells. Two years later he changed his view in an important way, now making a simple distinction between reproductive tissues, where free cell formation was the rule, and vegetative tissue, where cell division was the norm.
In 1845 Nägeli turned to the study of vegetable growth, his investigations culminating in the late 1850s when he traced the lineage of cells to a single apical cell. It was Nägeli who showed the regular way in which the original cell cuts off daughter cells—in either one, two, or three rows—which gave him laws that could be represented mathematically. In his later studies, Nägeli conceived the important distinction between formative tissue ( Bildungsgewebe ) and structural tissue ( Dauergewebe ) which is no longer actively multiplying. He observed that in the stems and roots of plants there was a certain type of cell that remained unaffected by differentiation and whose origin could be traced back all the way to the original “foundation cell” or zygote. This distinction between formative and structural tissue was an early sighting of the idea of heredity. 41
To the end of his life, Nägeli continued to believe in the spontaneous generation of cells. And so, when Gregor Mendel sent Nägeli his Versuche über Pflanzen-Hybriden in 1866, Nägeli took Mendel’s work seriously enough to repeat it. Unfortunately for him, he used Hieracium , a plant that reproduced asexually, and he therefore thought that Mendel’s hybrid ratios and demonstrations of complete reversion, though of mathematical interest, were irrelevant to genuine species. Although Nägeli failed to recognize Mendel’s genius, his pupil Karl Correns was less blind. Correns was one of the three rediscoverers of Mendel’s laws. 42
Observations on the formation of the embryo within a fertilized hen’s egg were made more than 2,000 years ago, before Aristotle. But the link between cells and embryos was not really made until 1827, when Karl Ernst von Baer reported his observations about the mammalian ovum in a (Latin) letter written from Leipzig. For obvious reasons, study of the development of a single mammalian egg was virtually impossible, let alone a single human egg. The first description of segmentation in the egg was not made until 1824, by the Frenchmen Jean-Louis Prévost and Jean Baptiste Dumas. They recognized that the furrows they observed deepening on the surface of the developing egg were the first signs of its division, and that this process was repeated until the structure came to look “like a raspberry.” 43 Difficult as it is for us to understand now, it never crossed their minds that what they were describing was cell multiplication.
Only in 1834 did von Baer publish his more
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