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 Z-celleO, -4-celIeB. 



clecrvage 

 ^ cavity- 



ectoSLerm 



l^-cellecl 



tnout-h 



primitive 



32Celle3l "hollovlxxll- beginning tvo^lccyere^ 



embr^ blastula of gdstrula g*cc^-br\xlcc 



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- 

 ner laver the endoderm. A mid- 

 die layer, the mesoderm, soon 



speTin 



pt 



fertile e^g 

 deavag'e 

 blastixla 

 gccstrulcc 



I V 



appears between the ectoderm 

 and the endoderm. The folded 

 edges of the gastrula grow 

 toward each other, forming a 

 tin}^ mouth. IMarked differ- 

 entiation now starts and forms 

 the characteristic tadpole. 

 The skin and nervous system 

 circulotory ZfS^^I^^^^ are formed from the ectoderm, 

 fj. "I J. -l-he digestive and respirator}^ 



The fertUe egg, by repeated divisions and certain SystcmS are formed from the 

 changes, becomes the gastrula with its three pri- i i rpi , 



mary germ layers. Further differentiation and de- enQOQerm. lUC CXCrCtOry, TC- 



velopment goes on and the organ systems arise. productive, mUSCUlar, skeletal, 



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

 ment the yolk is entirel}^ absorbed by the actively dividing cells- 



ectocLerm inesoderm endoderm. 



epidermis muscles SLitfestive, 



J w T i. sv^tenv 



central excretory ,. ^ ^ 



nervous sy^ts-rrC nver anci 



s<X-^^^^ ^ncras l^^^^^s 

 receptor? 



