286 



REPRODUCTION OF ANIMALS 



cells known as the blastula. The blastula pushes in just as a rubber 

 ball may be dented in by pushing on one side. A double-layered, 



fertile egg 2-celleoL -4-celIeU 



lS-cellecl 



--mouOi 



32 CelteSt 



"hollov tall 

 blccstula 



fcegmnii _ 

 ofjSocgtrula 



"tvo lccyere& 

 g'ccs-br-ulcc 



The embryo begins as a single fertilized egg. This divides to form two, then four, and cleav- 

 age continues until a great many cells are formed. Gradually differentiation sets in and special- 

 ized structures may be recognized. Thus the various organs of the organism develop. 



cup-shaped structure known as the gastrula is thus formed. The 

 embryo is now two-layered. The outer layer is the ectoderm, the in- 



sperm 



cleavage 

 blastutlcc 

 gbcst-rulcc 



ectdcLerm mesoderm endoderm. 



^ + *x, 



epidermis muscles 



central excretory 

 nervous sydtjem- 

 sx-stem J; na: as FWWWW 

 receptors drcuk ^ 



system/*" 



Skeleton 



The fertile egg, byrepeated divisions and certain 

 changes, becomes the gastrula with its three pri- 

 mary germ layers. Further differentiation and de- 

 velopment goes on and the organ systems arise. 



rt-em. 



ner layer the endoderm. A mid- 

 dle layer, the mesoderm, soon 

 appears between the ectoderm 

 and the endoderm. The folded 

 edges of the gastrula grow 

 toward each other, forming a 

 tiny mouth. Marked differ- 

 entiation now starts and forms 

 the characteristic tadpole. 

 The skin and nervous system 

 are formed from the ectoderm. 

 The digestive and respiratory 

 systems are formed from the 

 endoderm. The excretory, re- 



productive, muscular, skeletal, 

 and blood systems form from the mesoderm. During this develop- 

 ment the yolk is entirely absorbed by the actively dividing cells- 



