52 GENERAL BIOLOGY 



It should be clearly understood that the nuclear changes 

 described above constitute an uninterrupted process. The 

 separation of the process into distinct -phases or stages is 

 made for convenience in description and analysis. The prep- 

 arations on the slides which show the distinct phases, there- 

 fore, represent cells which were killed in the midst of the 

 process, and which were permanently fixed in this condition. 



4. Cleavage and Development. In order that an egg may 

 develop and grow into a new organism several preparatory 

 processes are essential. First a ripening or maturation 

 process is necessary for both the female reproductive cells 

 (ova or eggs) and the male reproductive cells (spermatozoa). 

 After maturation fertilization must occur, and this consists 

 in the union of an ovum and a spermatozoon. The fertilized 

 egg is now capable of further development which is initiated 

 by a division into cells, a process called cleavage or segmen- 

 tation. 



In dividing starfish eggs look for stages of 2-, 4-, 8- cells. 

 Are these cells enclosed within a membrane? Are they of 

 equal size? Each cell continues to divide until a large 

 number are present, and these are arranged in the form of 

 a hollow sphere, the blastula. Are all the cells of the blastula 

 of the same size? Look for other stages in which one side 

 of the blastula is flattened or pushed into the hollow of the 

 sphere. Such a stage is spoken of as the gastmla and is 

 really a double-walled sac or embryo, the outer wall mak- 

 ing up the ectoderm and the inner the entoderm. Later a 

 middle layer or mesoderm, is formed between the ectoderm 

 and the entoderm. These three layers are the germ layers 

 of the embryo from which are differentiated the organs of 

 the developing organism. 



Study and draw several stages of cleavage, of the blastula 

 and gastrula formation. 



