of the flat neural plate fold upward and fuse, forming the beginnings of the hollow brain and spinal cord. During differentiation cells mature so that they may perform separate functions. This maturation may include taking on a function-related shape, such as the long, spindly shape of skeletal-muscle cells. Cell differentiation also results in responsiveness-the ability of a cell to be regulated within the organism through the action of hormones, neurons, and other signals.
胚经过内部细胞和表面细胞特化后，器官形成，发育成器官和组织。器官形成实质上包括两个联系紧密的过程，即形态发生和分化。在形态发生期间，细胞和细胞群体形状发生改变：例如，脊椎动物的神经胚，扁平神经板的边缘向上折叠并融合，是中空大脑和脊髓神经形成的开端。在分化期间，细胞成熟，执行各自功能。成熟包括功能相关的成型过程。例如，细长的骨骼肌肉细胞。细胞分化也产生应答能力，即细胞受生物体内激素，神经和其他信号的调控过程。 Embryonic Coverings and Membranes
The embryos of land vertebrates are enclosed within four extraembryonic membranes that afford protection while still permitting the exchange of gases, nutrients, and other materials. 脊椎动物的胚胎被4层膜包被，起保护作用，但仍能交换气体，营养和其他物质。 Growth: Increase in Size
Growth in embryos is largely due to an increase in the number of cells rather than to an increase in the size of individual cells. In many species the extent of embryonic growth is limited by the availability of food (yolk). In animals that develop entirely free of the maternal body, such as frogs and insects, the embryo give rise to a larval stage that can feed itself and later undergo metamorphosis to attain the adult stage. In many species the most spectacular growth phase takes place during the juvenile and adolescent phases of the life cycle. Actual growth generally
stops once the organism reaches its typical adolescent phases of the life cycle. Actual growth generally stops once the organism reaches its typical adult size, although replacement of dead cells may continue.
胚的生长很大程度取决于细胞数量的增加而不是单个细胞大小的改变。在大多数物种中，胚的生长是受卵黄的量限制的。在动物细胞中，完全依赖于母体，例如蛙和昆虫，胚产生幼虫状态，后来经历变态而进入成年状态。在许多物种中，在少年和青春期，发生特别显著的生长。一旦细胞进入成年状态和成年大小，生长就停止了，尽管死细胞的替代不断发生。
A special type of growth, regeneration of lost body parts, can take place in adults of some species. Prior to such regeneration cells in stump tissue undergo dedifferentiation. They lose their functional phenotype, divide rapidly, and generate a population of cells that will regenerate the lost part. Compensatory hypertrophy is a different, temporary growth response in which residual tissue increases in mass and cell number: cells undergo mitosis but do not dedifferentiate.
一个特殊类型的生长，再生失去的部分躯体，发生在某些物种的成体中。在再生之前，残肢组织细胞经历了去分化过程。他们失去功能表型，快速分裂，产生细胞群来再生失去的部分。代偿式肥大是一个不同的，暂时的生长应答过程，残余组织在体积和细胞数量上增加：细胞经历了减数分裂，但没有去分化过程。
Aging and Death: Final Developmental Processes
Aging is an ongoing, time-dependent developmental process in which body parts deteriorate. Proposed causes include the degeneration of collagen (the fibrous proteins of the connective tissues) and limits on the number of times cells can divide. Other theories focus on a decline of the immune system or on the accumulation of lipofuscins (aging pigments).
老化是一个持续过程，随时间发育，身体部分恶化。可能原因包括胶质的退化（纤维蛋白）和能分裂活细胞的限制。另外理论认为，免疫系统的免疫下降或脂褐素的堆积造成的。 课后习题：阅读材料一
Glossary
sperm精细胞
Cells that develop from the spermatids by losing much of their cytoplasm and developing long tails; the male gamete.
ovum 卵，卵子（复数,ova）
An unfertilized non-motile female gamete. In many animals it is produced in the ovary. spermatogenesis 精子发生
The specific name given to the gametogenesis process that leads to the formation of sperm. oogenesis 卵子发生-
The specific name given to the gametogenesis process that leads to the formation of eggs. ovaries 卵巢，子房
The female sex organs the produce haploid sex cells, the eggs or ova.
yolk 卵黄，蛋黄
The store of food material, mostly protein and fat, that is present in the eggs of most animals oviduct 输卵管
The tube (fallopian tube) that carries the primary oocyte to the uterus.
gene amplification 基因扩增
A temporary dramatic increase in the numbers of a particular gene in a genome during one developmental period.
fertilization 受精
The joining of haploid nuclei, usually from an egg and a sperm cell, resulting in a diploid cell called the zygote. Zygote受精卵
A diploid cell that results from the union of an egg and a sperm.
acrosome reaction 顶体反应
That series of observable, structural changes undergone by a sperm when in the vicinity of an ovum in the oviduct. Specifically, many openings appear in the sperm head membrane, through which the contents of the acrosome appear to be released.
parthenogenesis ['pa:einE]u'd3enisis]单性生殖，孤雌生殖
Reproduction in which eggs develop normally without being fertilized by a male gamete, producing an individual usually genetically identical to the parent. cleavage 卵裂
The mitotic division of the zygote that occurs
immediately after fertilization and produces a ball of smaller cells without an overall increase in size. blastula 囊胚，囊胚泡
The stage of the early animal embryo that succeeds cleavage and precedes gastrulation. A blastula is
usually made up of a hollow ball of cells, with a wall one to several cells thick; the central cavity is the blastocoel. blastomere 卵裂球
One of several small cells formed from an animal zygote during cleavage.
gastrulation 原肠胚形成
The process in the animal embryo by which a blastula is converted into a gastrula, with the laying down of the germ layers.
gastrula 原肠胚
The stage in the development of animal embryos that follows the blastula and results from gastrulation. The cells of a gastrula are differentiated into the germ layers and the central cavity opens to the exterior by the blastopore.
ectoderm 外胚层，内层
The outermost of the germ layers of metazoan embryos. It develops mainly into epidermal tissue, the nervous system, sense organs, and (in lower forms) the nephridia. endoderm 内胚层，内层
The innermost of the germ layers of metazoan embryos. Mesoderm中胚层，中层
The middle layer of the three germ layers of triploblastic animal embryos. It develops into cartilage, bone, muscle, blood, kidneys, and gonads. primitive streak 原肠胚
A longitudinal band of embryonic mesodermal cells that develops within a dorsal groove along the length of the gastrula of mammals and birds. organogenesis 器官形成 The formation of organs.
morphogenesis形态发生，形态建成
The developmental processes leading to the characteristic mature form of an organism or part of an organism. differentiation 分化，鉴别
The complex of changes involved in the progressive diversification of the structure and functioning of the cells of an organism. For a given line of cells,
differentiation results in a continual restriction of the types of transcription that each cell can undertake. neurulation 神经胚形成
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