94 PHYSIOLOGY 



out of the mesoderm, everything else, — bones, muscles, 

 blood, connective tissue, glands. 



The Process Universal. — The development of the sea- 

 nrchin up to the gastrula is typical. The main changes 

 that have been described as taking place during fertiliza- 

 tion, cleavage of the egg, and formation of the first two 

 germ layers occur in all multi-cellular animals. Each 

 particular group of animals, however, has its own 

 peculiarities of development resulting from variations 

 in environment or structure. Some animals like the 

 hydra never pass beyond the gastrula stage. Others 

 more highly developed pass through all of the stages of 

 their immediate ancestry. In other words, " the history 

 of the individual repeats the history of the race." This 

 fact is one of the strongest arguments in favor of the 

 theory of evolution; it seems to justify the assumption 

 of a common ancestry, and belief in the evolution of 

 complex from simple forms. 



Protection of the Cells in Land Animals. — In land 

 animals egg cells and spermatozoa can not be turned 

 loose because their delicate outer walls would dry in the 

 air and the cells would die. Some way must therefore 

 exist by which the sperm cell may reach the egg cell 

 without danger of the death of either one. The cells 

 reach the exterior of the sea-urchin through tubes that 

 lead from the organs where the cells are formed to the 

 outside. These tubes are moist. In land animals the 

 cells come in contact with each other without exposure 

 to the air in one of the tubes. 



As the egg cell can not move and the sperm cell can 

 move, the sperm cell passes over to the tube containing 



