Embryology is the study of the individual development of living organisms, but the field it encompasses is now very broad. Initially, embryology was the study of the morphological observation and description of the developmental process of various animals (descriptive embryology). Because the embryo, which is in the early stages of development, is tiny, the progress of research owes much to the development of microscopes at the time. Furthermore, by using oviparous organisms such as the eggs of marine animals, morphological research has become possible in great detail. In the 19th century, the developmental processes of various animals began to be compared (comparative embryology). It was discovered that there were aspects of the developmental process of animals that differed between different species, as well as commonalities. For example, all animals pass through the blastula stage during development. This led to the idea that the development of an individual reflects a certain historical process. One such idea was E. H. Haeckel's famous recapitulation theory, which claims that individual development is a shortened repetition of phylogenetic development. Research into the morphology of larvae and homologous organs deepened consideration of the evolution of living organisms, and helped to advance the theory of evolution. In the 20th century, there was a movement to clarify the causes and effects of developmental processes by not only observing embryos but also performing experimental manipulations and examining the results. This movement was started by W. Lew, who is said to be the father of experimental embryology. During this period, techniques were established to freely cut out and implant amphibian embryos using fine glass needles and hair loops. The main questions at the time were what each part of the embryo could differentiate into, and how it influenced the others (sometimes called developmental mechanics). Spemann discovered that when he cut out the upper lip of the blastopore and implanted it into another embryo, the embryo would produce an extra embryo in addition to the original embryo, and that this was entirely due to the function of the implanted upper lip of the blastopore, and he named this part the organizer. This led to the establishment of a view of embryo development as a series of induction chains. More recently, there has been a dramatic increase in knowledge about cells, and with the rapid development of biophysical chemistry and molecular biology, developmental biology is once again on the verge of transformation. For example, there is a new approach that regards the embryo as a social group of cells, and attempts to understand the mechanism of the formation of an individual organism by analyzing the information exchange there. [Kinoshita Seiichiro] Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend |
生物の個体発生を研究する学問であるが、これに含まれる分野は現在非常に広いものとなっている。発生学は最初さまざまの動物の発生過程の形態的観察とその記載をする学問であった(記載発生学)。発生の初期にある胚(はい)は微小なものであるから、研究の進展は当時の顕微鏡の発達に負うところが大きい。また、海産動物の卵など卵生のものを用いることによって、形態学的研究はきわめて詳しく行われるに至った。 やがて19世紀になると、種々の動物の発生過程が比較されるようになる(比較発生学)。動物の発生過程には、種類が違えば異なっている部分と、共通にみられる部分とがあることがわかってきた。たとえば、すべての動物が発生の途中でかならず胞胚の時期を経る。こういうことから、個体の発生はある歴史的な過程を反映しているものであるという考えが生まれた。その一つにE・H・ヘッケルの有名な反復説がある。個体発生は系統発生の短縮された反復である、というのがその主張である。幼生の形態や相同器官などの研究から、生物の進化についての考察が深まり、進化論を推進することとなった。 20世紀に入ると、胚を観察するばかりでなく実験的操作を加えてその結果を調べ、発生過程のなかでの原因と結果とを明らかにしようとする動きが始まった。その始まりはW・ルーで、実験発生学の祖といわれる。この時代に両生類の胚を細いガラス針や毛髪のループ(輪)などを使って自在に切り出したり植え込んだりする技術が確立された。胚の各部分が自身では何に分化しうるのか、他に対してはどのような働きかけをしているのか、などが当時の主要な命題であったが(発生機構学とよぶことがある)、シュペーマンは、原口上唇を切り出してほかの胚に植え込むと、その胚には本来の胚のほかに余分にもう1個の胚を生ずること、それはまったく植え込まれた原口上唇の働きによることをみいだし、この部分を形成体と名づけた。これに端を発して、胚の発生を一連の誘導連鎖としてとらえる見方が確立した。 さらに最近に至って、細胞についての知見が飛躍的に増加し、加えて生物物理化学、分子生物学も急速に発展したため、発生学はふたたび変貌(へんぼう)しようとしている。たとえば、胚を細胞の社会集団としてとらえ、そこでの情報のやりとりを解析して有機体としての個体の形成の仕組みに迫ろうとするものなどは、その一つである。 [木下清一郎] 出典 小学館 日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について 情報 | 凡例 |
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