SEXUAL REPRODUCTION AND DEVELOPMENT OF INDIVIDUAL 2l5 



As cell divisions continue in the blastula, a gastrula is finally 

 formed. The blastula does not simply increase in circumference, 

 but there comes a time when the wall on one side pushes in (in- 

 vaginates), finally meeting the wall of cells from the other side. 

 This gradually crowds out the cavity and forms a wall of two layers 

 of cells. The outer layer is known as the ectoderm (outer skin) and 

 represents the portion of the wall of the blastula which has not 

 folded in. The inner layer, or that resulting from the infolding of 

 the wall of the blastula, is called endoderm (inner skin). As divi- 

 sion of cells in this wall proceeds and the infolding continues, the 

 two margins of the infolded part come nearer and nearer each other. 

 This gradually encloses an outside space which is lined by the 

 endoderm and represents the primitive digestive tract or archenteron. 

 This is the beginning of the two primitive genu layers, ectoderm and 

 endoderm. In sponges and coelenterates development stops here. 



In higher forms, immediately following gastrulation, a third germ 

 layer, the mesoderm (middle layer), is organized from cells usually 

 contributed by one or the other or both of the other germ layers. 

 In some cases it arises as two saclike outgrowths from the endoderm, 

 one on each side in the gastrula. These pouches push into the 

 remains of the blastocoele. In other cases separate cells are shed 

 from ectoderm or endoderm or both, or from an undifferentiated 

 portion to organize as a distinct layer between the other two. The 

 position of the mesoderm is external to the' endoderm and internal 

 to the ectoderm. It nearly encircles the endoderm. Sooner or later 

 a space forms within the mesoderm, causing the outer limb of it to 

 join the ectoderm and the inner to join the endoderm. This cavity 

 is the coelom or future body cavity. From each of the germ layers, 

 particular parts of the body are derived. 



The fate of the geryn layers is determined as cell division and 

 development continue. Cell division proceeds at different rates in 

 different regions and at different times, resulting in various infold- 

 ings, outpushings, and extensions which finally bring about the 

 formation of all parts of the mature individual. The ectoderm gives 

 rise to the external surface cells or epidermis of the skin and to 

 the nervous system ; the mesoderm furnishes the muscles, skeleton, 

 circulatory system, blood, excretory, and reproductive systems be- 

 sides nearly all connective tissue ; and the endoderm produces the 

 internal linings of the digestive tract, respiratory tract, and such 

 outgrowths as the liver and pancreas 



