胚胎(embryo)
动物在卵中或母体子宫内发育的早期阶段，在这段期间其基本的形式、器官和组织开始发展。在人类指受孕後到第七或第八周为止的有机体，之後则称为胎儿（fetus）。在哺乳动物中，卵与精子结合而成受精卵，受精卵发生数次细胞分裂（卵裂）後，形成一个中空球形的囊胚。受精之後第二周，原肠胚形成後，胚胎分化为三个组织类型，这三种组织类型再发展为不同的器官系统︰外胚层发育成皮肤和神经系统；中胚层发展为结缔组织、循环系统、肌肉和骨骼；内胚层形成消化系统、肺以及泌尿系统。以人类来说，在受孕後第四周左右，头部和躯干的区分逐渐明显，脑、脊髓、内部器官开始发育。到了第五周，四肢开始出现，胚胎长约0.33寸（0.8公分）。到第八周结束，胚胎已成长到1寸（2.5公分）长，接下来的变化有限，主要是现存结构的成长和特化。任何的先天性疾病（congenital disorder）也是在这个阶段就开始出现。亦请参阅pregnancy。
English version：
embryo
Early stage of development of an organism in the egg or the uterus, during which its essential form and its organs and tissues develop. In humans, the organism is called an embryo for the first seven or eight weeks after conception, after which it is called a fetus. In mammals, the fertilized egg or zygote undergoes cleavage (cell division without cell growth) to form a hollow ball or blastocyst. During the second week following fertilization, gastrulation (cell differentiation and migration) results in the formation of three tissue types. These three types of tissue develop into different organ systems: the ectoderm develops into the skin and nervous system; the mesoderm develops into connective tissues, the circulatory system, muscles, and bones; and the endoderm develops into the lining of the digestive system, lungs, and urinary system. In humans, by about the fourth week, the head and trunk can be distinguished and the brain, spinal cord, and internal organs begin to develop. By the fifth week, limbs begin to appear and the embryo is about .33 in. (.8 cm) long. By the end of eight weeks, the embryo has grown to about 1 in. (2.5 cm) long and all subsequent change is limited primarily to growth and specialization of existing structures. Any congenital disorders begin in this stage. See also pregnancy.
