Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/178

128 from the yellow yolk surrounding it. We have tried to explain how, by a continued process of cell-division, the contents of the egg of a mammal assume a mulberry-shaped form, and the gradual conversion of this mulberry into the External and Internal blastodermic membranes. If the yolk of the chicken be examined before it is surrounded by the semi-fluid substance (Fig. 173) and shell, there will be found lying on the top of the yellow yolk a membrane (Fig. 173, b) in which may be seen the Germinal vesicle and Germinal spot; by a process of cell-division, known as partial segmentation, this membrane is transformed into a heap of balls which gradually assume the form of the two membranes which, we have stated, are found lying upon the yellow yolk of the freshly-laid egg. In the course of development a third membrane appears between these two. The partial fusion of these membranes makes the Primitive trace, which passes from the oval to the guitar-shaped form (Fig. 177). The Middle membrane splits into two layers (Fig. 175), the Upper uniting with the External blastodermic membrane, the Lower bending down on the Internal blastodermic membrane (Fig. 176, c). The Middle and Internal blastodermic membranes now grow gradually downward around the yellow yolk, and finally inclose it. At this stage the embryo chick corresponds to Fig. 177, representing the embryo of a mammal. We see, therefore, that the development of the chick and the mammal is the same, while the difference between their eggs is not an essential one,—the nutriment for the mammalian egg being furnished from time to time, while that for the bird's egg is supplied at once in the form of yolk. A homely illustration of this difference is that of a man who receives his yearly food from day to day, and of one who receives his yearly food at once. The only part of the chicken's egg which corresponds to the mammal's egg is the membrane with its Germinal vesicle