Читать книгу Human Developmental Biology - Danton Inc. O'Day - Страница 12

At this point it is important to note an important and classic aspect of development raised by this material. In many lower organisms, the development of tissues and organs is determined as early as oogenesis. If parts of the egg are removed or specific blastomeres are removed in marine invertebrates for example, specific organs may not form. This type of development is called mosaic development. The fate of most if not all cells and tissues is established when the egg is formed. The formation of germ plasm is one example of the determination factors set up in such embryos. Humans and other mammals are at the other end of the developmental scale. The formation of human tissues and organs occurs in a sequential series of steps with each step defining the next. If a part of the embryo is removed during early development, the embryo can often survive and develop normally. If two blastomeres are separated, each can form an embryo. This is called regulative development. Many animal species fall between the two ends of the spectrum of mosaic and regulative development. The important topic of determination of cells and tissues and cellular interactions (e.g., induction) that mediate the process will be covered at many times throughout this book.