206 



LIFE S BEGINNING ON THE EARTH 



aspect if we consider the wealth of forms which may develop 

 from simple salt-crystals in the artificial osmotic structures. 

 The germ-cell is certainly more chemically complex than a 

 plain salt; and it does seem likely that the chemical make-up 

 is responsible for the wealth of its development. 



The first stages of development of the fertilized ovum are 

 much the same in all animals. The egg-cell, immediately 

 after fertilization, surrounds itself with a protecting en- 

 velope. Inside of this envelope, it divides in two, then in 

 four, then in eight, then in sixteen smaller cells and so for- 



M 

 73 



_C3 



3 

 ■*? 



<n 

 03 



m 



03 



a 



Fig. 79. Diagram of the Initial Stages of the Development of an 



Egg-cell 



The diagram illustrates the successive stages of development; first, 

 consecutive cell division by which the number of cells is increased from 1 to 

 2 to 4 to 8 to 16 to 32 to 64, etc.; second, the arrangement of the smaller 

 cells thus formed into a blastula and subsequently into a gastrula, which is 

 the initial form in the development of an animal. (Magnified 200 times.) 



ward. The small cells thus formed group themselves to 

 form a primitive organism, which soon develops further. 

 In the lower types of animals that live in the ocean, both 

 male and female shed their germ-cells, ovum and sper- 

 matozoon, directly into the ocean. Fertilization occurs due 

 to the motility of the spermatozoon, which may hit upon an 

 ovum, in this case the details of the process of fertiliza- 

 tion can be readily watched under the microscope. The 

 germ cells of sea urchins, starfish, or other invertebrate 

 animals lend themselves particularly well to such experi- 

 ments. All we need do is take some ova from the ovary of 



