302 REPRODUCTION IN ANIMALS 



be the case, in others it is quite the contrary. Evidently 

 the two processes are not so closely associated as in 

 higher types, but the subject requires further in- 

 vestigation. 



Early Development. — The union of two germ-cells, 

 as has been pointed out, results in every animal in a 

 single-celled fertilized egg. Beyond this point its devel- 

 opmental path depends upon the kind of organism it is, 

 although there are certain fundamental resemblances 

 throughout. 



In the starfish, which we shall use as an example, the 

 fertihzed egg divides by the ordinary process of division 

 into two cells of equal size (Fig. 72). These again 

 divide into four, these into eight, and so on until 

 hundreds of cells of nearly equal size are produced. Dur- 

 ing this segmentation period, as it is termed, when the 

 cells are dividing without forming any definite organs, 

 a cavity appears in the midst of the cells, and increasing 

 in size as the cells increase in number, becomes the hollow 

 of a spherical one-layered group. The young starfish or 

 embryo resembles a tennis ball, and at the end of thirty- 

 six hours has attained the blastula stage. 



The Gastrula and Body Layers. — With the increas- 

 ing division of the cells, the blastula becomes somewhat 

 flattened on one side, and is finally pushed inward in much 

 the same manner as one pushes in the side of a hollow 

 rubber ball. The cup-like depression deepens and finally 

 becomes a comparatively slender sac known as the primi- 

 tive gut or intestine. The embryo is now in the gastrula 

 stage. 



At about this time certain cells, appearing like minute 

 amoebae, migrate from the tip of the primitive gut and 

 wander about in the fluid within the cavity between the 

 gut and outer cells. The embryo now comprises an outer, 

 inner, and middle cell layer, respectively named the ecto- 

 derm, endoderm, and mesoderm. To anticipate, it may be 

 said that the ectoderm gives rise to the skin, sense organs. 



