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5 The Descriptive Embryology of Male and Female Development The study of embryology was extremely limited until the 1860s, when several techniques came together. One was the development of achromatic lenses, which allowed scientists to observe cells or other small objects without the confusion of chromatic aberration and spherical aberration .Chromaticaberrationinvolvesthebreakingupofwhitelightinto a rainbow of concentric colors around an object. Spherical aberration is even worse because there is no central focus and competing points of convergence lead to a blurred image. Joseph Jackson Lister (1786–1869), whosemorefamousson,JosephLister(1827–1912),introducedantiseptic surgery, provided the solution.1 Like his father, Lister was a wine merchant. They were Quakers and fairly pious. Lister worked after hours on his interest in natural history. When he took up microscopy, he found the interference with sharp images annoying and switched to making his own lenses. Lister found that dissolving lead and other salts into glass changed the density within these glasses, which yielded different focal lengths when he made lenses from them. He eventually worked out a combination of two layers of molten glass (crown glass and flint glass), one correcting the other for bothchromaticandsphericalaberration.Theresultingachromaticlenses revolutionized both astronomy (allowing bigger telescopic lenses) and biology(allowingcompoundmicroscopesthatwouldmultiplyanobject up to 2000 times. He began his experiments on lenses in 1824 and found a suitable achromatic lens in 1826, which he refined until publishing his findings in 1830. The Embryology of Male and Female Development 27 The second advance in microscopic studies was the invention of stain technology.2 Even with achromatic lenses, the images of cells were faint; only subtle shades of gray distinguished the contents of individual cells or cells of one tissue from cells of another tissue. Several attempts to dye cells failed before Joseph von Gerlach (1820–1896) made a breakthrough by accident. In 1857, he made a slide containing some neural tissue with an acetocarmine dye on a preparatory plate and forgot about itwhenhewenthome.Whenhecameinthefollowingdayandwasabout to clean the slide and start over, he decided to look at it first. What he saw was a very distinct coloring of the nucleus of the cells and a very pale pink of the cytoplasm. The clarity was a substantial improvement over anycellshehadexaminedbefore,whichledtoaflurryofworkasheused his technique on different tissues and described them in 1858. Thethirdinnovationwasamachinethatcouldproducethinsections of material that later could be dyed. In 1665, Robert Hooke (1635–1703) used a razor to cut thin sections of a plant stem or to dissect small animals like worms and insects when preparing specimens for his microscope , but it was not until the 1770s that machines, called microtomes, wereintroduced.Earlymicrotomeswerecrankedbyhand;mostofthem were subject to vibrations and produced uneven slices. In 1835, Andrew Prichard (1804–1882) introduced a table model that was relatively free of vibration. Prichard’s design was much improved by Wilhelm His (1831–1904) in 1865.3 Additional innovations included the use of various solvents (water, alcohol, xylene, liquid paraffin, etc.) and Coplin jars to house the slides, and resins, like liquid balsam, to permanently cover the slides with a small glass cover slip. Many new dyes were introduced after William Henry Perkin (1838–1907) discovered aniline dyes in 1856 (at the age of 18).4 These innovations were refined in the 1870s and 1880s. From 1865 to 1871, Rudolf Heidenhain (1834–1897), Carl Weigert (1845–1904), and many others contributed to using multiple dyes, like eosin and haemotoxylin , to differentially stain the nuclear and cytoplasmic contents. Haemotoxylin is a dye extracted from logwood, a tree found in the Caribbean islands and Central America. It stains the nuclei deep blue or purple. Eosin is a fluorescent, aniline dye that stains the cytoplasm light 28 pink. By the 1890s, the eosin and haemotoxylin combination became the most widely used histological and cytological staining technique. Real advances in embryology took place when all of these componentscametogetherinthelastquarterofthenineteenthcentury .Chick, frog, and pig embryos were cut into paraffin ribbons like salami slices joined end to end. This was done for chick embryos at 24-hour, 48-hour, 72-hour, and 96-hour stages of development, and the fate of each of the three embryonic layers was followed: the ectoderm produced skin and thenervoussystem;theendodermproducedtheguts,lungs,andtrachea as well as pancreas and liver; and the mesoderm produced the bulk of the embryo including the muscular, skeletal, urogenital, and vascular systems. From Epigenesis to Preformation and Back to Epigenesis Aristotle trusted his eyes; when he broke open chick eggs, he saw the gradualemergenceof structure(form) fromauniformmatter thathe associated with blood (clear, then red, and then flesh...


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