336 



ANIMAL BIOLOGY 



cleavage by which the egg is divided into a number of 

 cells. Typically cleavage results in the formation of a 

 hollow sphere of cells, the blastula; this becomes pushed in 

 on one side forming a double-layered sac called the gas- 

 trula whose inner wall generally gives rise to the lining 

 of the digestive cavity and the various organs that arise 

 from it. In some animals, especially those in which 

 the egg contains much yolk, the blastula and gastrula 

 stages may become very much modified, so that they are 



4 



FIG. 228. Early development of Amphioxus. i, four cell stage; 2, 

 eight cells; 3, 16 cells; 4, 32 cells; 5, blastula; 6, blastula cut through the 

 middle; 7, early, and 8, late gastrula. (Prom Hatschek.) 



not easily recognized, but in most of the multicellular 

 animals they are nevertheless represented in some form. 

 In the gastrula stage there are formed the two primary 

 germ layers, an inner, the entoderm, and an outer, the 

 ectoderm. In animals above the ccelenterates a third 

 germ layer, the mesoderm, arises between the two others. 

 At first the cells of the embryo are similar in kind, but, 

 as the organs develop from the germ layers, the cells 

 become gradually differentiated into all the varied kinds 

 found in the adult body. 



