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ELEMENTARY BIOLOGY 



formation of new cell layers and with changes in the form of 

 the young embryo there gradually arise new kinds of cells. 

 At first these are in layers, or membranes. We may say that 

 the embryo at one time consists of membranes and cavities. 

 These membranes grow out irregularly into the cavities, form- 

 ing folds ; they break through in places, and they unite in other 



places. In this way there 

 appear the tissues and the 

 organs that make up the 

 young animal. 



327. Stages in develop- 

 ment. If we make a closer 

 comparison of the devel- 

 opment of a number of 

 animals, some remarkable 

 facts appear. In the be- 

 ginning, we may say that 

 all animals are like the 

 protozoa that is, each 

 one consists of a single 

 cell. Now if we take a 

 large number of higher 

 animals, like the starfish, 

 the snail, a primitive, fish- 

 like animal called the lancelet (Amphioxus), and others, we 

 shall find that in the development of each there is reached a 

 stage consisting of a hollow sphere of cells, the "mulberry" 

 stage, or the mom la, which is shown in Fig. m,/. This 

 hollow ball can be very well compared to such an organism 

 as the volvox (Fig. 113). In the development of the frog, 

 birds, and many other animals this stage does not appear so 

 clearly, because the presence of the yolk obscures the symmetry 

 of the hollow sphere. 



When the hollow sphere caves in and the opposite sides 

 meet, forming a two-layered cup (Fig. in, g-ti), this stage of 



FIG. 113. Volvox 



This organism consists of a hollow sphere made 

 up of a single layer of cells connected by strands 

 of protoplasm. The colony moves about in the 

 water by means of cilia, or vibrating protoplasmic 

 threads. Each cell contains chlorophyl 



