. Cunningham's Text-book of anatomy. Anatomy. Fig. -Segmentatk in i if Zygote. 4-Cell Stage. Segmentation. Immediately after its formation the zygote is separated, by a series of consecu- tive mitotic divisions, into a large number of cells which are grouped together in the form of a solid spherical mass, called a morula on account of the mulberry- like appearance of its surface. This period of division is called the period of segmentation (Figs. 24-27). The segmentation divisions are of the homotype form, and there is evidence which tends to the con- clusion that the earliest divisions,
. Cunningham's Text-book of anatomy. Anatomy. Fig. -Segmentatk in i if Zygote. 4-Cell Stage. Segmentation. Immediately after its formation the zygote is separated, by a series of consecu- tive mitotic divisions, into a large number of cells which are grouped together in the form of a solid spherical mass, called a morula on account of the mulberry- like appearance of its surface. This period of division is called the period of segmentation (Figs. 24-27). The segmentation divisions are of the homotype form, and there is evidence which tends to the con- clusion that the earliest divisions, by which the zygote is divided first into two and then into four parts, are quantitatively and qualitatively equal. After a time, however, the divisions result in the formation of cells of different sizes and different capabilities, definite and circumscribed functions being allocated to certain groups of cells and their descendants. It is probable that at this time cells are set apart which are the pro- genitors of the germ cells of the next generation, and which therefore retain all the capabilities of their ancestors. These cells are the means by which the species is reproduced and the hereditary tendencies are transmitted from generation to generation. At the same time other cells are set apart for the production of the tissues and organs of the individual which will be produced from the zygote, and in which the germ cells and their descendants will be lodged and protected till they attain their maturity. After the morula is established one of the first definite changes which occurs in its constitution is the differentiation of its cells into an outer layer and an inner mass (Fig. 26). In the human subject, as in many other mammals, the cells of the outer layer constitute the trophoblast or trophoblastic ectoderm, which plays a most im- portant part in the nutrition of the embryo and foetus. They enter into the formation of the chorion, or outer- most envelope of the growin
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectanatomy, bookyear1914
