= Solution
\b[Nineteenth-century <embryology> transformed knowledge of <reproduction> from an account of fluids and visible growth into an increasingly cellular account of <fertilization> and <biological development>. It did not end with a complete mechanism of <heredity>.] The century must be treated as a changing sequence: what was established by the 1890s cannot be attributed without qualification to an investigator in 1800. The key changes concerned the material contributions of the parents, the origin of embryonic structures and the evidential power of observation versus intervention.
An early problem was identifying the mammalian <ovum>. A large ovarian follicle is not itself the <ovum>, and failure to distinguish the two could confuse the material starting point of <biological development>. In 1827 <Karl Ernst von Baer> described the mammalian <ovum>, notably from the dog, in <De ovi mammalium et hominis genesi by Karl Ernst von Baer>. Dissection located the relevant structures; <optical microscopy> distinguished the small <ovum> within the follicle. The evidence was therefore a combination of anatomical placement and magnified appearance. The work's title must not be taken as proof that he directly observed every corresponding stage in human <reproduction>.<De ovi mammalium et hominis genesi by Karl Ernst von Baer>[1827 account and accompanying plate of the mammalian ovum, including the observations on the dog]{ref}
This mattered to the contest between <preformationism> and <epigenesis>. If the <embryo> were already a tiny formed individual, growth would chiefly enlarge structures already present. Successive observations of incubated eggs instead showed structures appearing and differentiating over <time>. Serially examining <embryos> at different stages made an ordered developmental account possible; it did not normally follow the same opaque mammalian <embryo> continuously from beginning to end. Nor did rejection of a miniature adult settle every question about invisible inherited organization. An investigator could accept the visible emergence of structures yet propose pre-existing determinants below the resolving power of the <optical microscope>.<The Cell by Oscar Hertwig>[Chapter IX, sections I–II: older and more recent theories of development and reproduction]{ref}
During the century, <cell theory> and improved techniques made <cell division> a central explanatory object. Lenses, illumination, thin sections, fixation and <staining> helped observers distinguish <cell nuclei>, <chromosomes> and patterns of division. These were achievements of preparation as well as magnification: a preparation could preserve a fleeting structure but also distort it. Comparing preparations and checking fixed material against living specimens were therefore important safeguards. By the late century, a developmental sequence could be described as successive divisions and rearrangements of <cells>, rather than the spontaneous appearance of a fully organized body from undifferentiated fluid.<The Cell by Oscar Hertwig>[Chapters I–II on investigating and organizing the cell; chapter VI on cell multiplication and nuclear division]{ref}
<Fertilization> became a particularly revealing test. External <fertilization> in transparent marine eggs allowed investigators to mix <sperm> and unfertilized <ova> at a known <time> and watch the sequence under an <optical microscope>. <Oscar Hertwig>'s sea-urchin investigations in the 1870s connected <sperm> entry with the appearance and union of the male and female nuclear contributions before the first <cleavage division>. His later <The Cell by Oscar Hertwig> presents these events as central to the process. Separating eggs from <sperm>, observing them before mixing, and comparing the sequence after mixing could distinguish <fertilization> from ordinary changes in an ageing egg. Serial nuclear observations supplied evidence about what entered and what happened next, not merely a correlation between mating and later offspring.<The Cell by Oscar Hertwig>[Chapter VII, section I.1(a), Echinoderm Eggs: sperm entry, the sperm and egg nuclei, and their relation to the first division]{ref}
Nevertheless, fusion of parental nuclear material did not reveal the modern molecular mechanism of <heredity>. <Chromosomes> supplied observable structures whose behavior could be compared across <cell divisions>, and the contributions of the two <gametes> could be investigated. This is different from knowing that <DNA> encodes information in a particular <nucleotide sequence>, or knowing how that information directs development. Even the apparent continuity or disappearance of a <cell nucleus> during division was a historically difficult observational problem. The distinction between a visible structure and the theoretical function assigned to it is essential.<The Cell by Oscar Hertwig>[Chapter VI, account of competing earlier interpretations of nuclear division; chapters VII and IX on fertilization and theories of heredity]{ref}
The final decades added experimental <embryology>. In 1888 <Wilhelm Roux> injured one of the first two <blastomeres> of a frog <embryo> with a heated needle; partial development of the survivor appeared to support a <mosaic theory of development>, in which early divisions distribute distinct developmental capacities. In 1891 <Hans Driesch> separated early sea-urchin <blastomeres> by shaking and found that an isolated <blastomere> could form a small complete larva. That showed a regulatory capacity inconsistent with an exclusively rigid allocation of half the future <organism> to each of the first two <cells>.<The Science and Philosophy of the Organism by Hans Driesch>[Part I, section B.1, Experimental Morphology: The Work of Wilhelm Roux and The Experiments on the Egg of the Sea-urchin; retrospective account explicitly dating the experiments to 1888 and 1891]{ref}
The comparison requires care: killing a <cell> and leaving it attached is not the same intervention as removing it, and a frog is not a sea urchin. The results therefore did not establish that all <embryos> develop identically or that every early <cell> can make an entire <organism>. They did establish that a normal developmental fate cannot automatically be equated with an irreversible developmental capacity. <Hans Driesch>'s later commitment to <vitalism> was one interpretation of this capacity, not something a complete larva logically proved. His 1908 retrospective account is valuable participant evidence, but it is later than the nineteenth-century trials it describes.<The Science and Philosophy of the Organism by Hans Driesch>[Part I, section B.1: discussion of isolated blastomeres, regulation and the limits of the mosaic interpretation; Preface identifies the lectures as a personal philosophical account]{ref}
\b[By the century's end, investigators had strong evidence for the cellular starting points of <sexual reproduction>, parental nuclear participation in <fertilization> and active organization during <biological development>.] They knew these things through dissection, prepared and living microscopic observations, comparison of stages, and increasingly discriminating interventions. Their continuing uncertainty about <heredity> and developmental organization was productive: it concerned mechanisms that their best observations had made more precisely questionable, rather than a simple absence of facts.
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