Reproduction and Development. 53 



which may be ciliated, so that by the lashing of these 

 cilia it is enabled to swim freely in the water. The inner 

 cavity is the primitive digestive cavity. 



A cross-section through the middle of the embryo at 

 this stage will show this central cavity surrounded by a two- 

 layered body- wall (H.). A little later the following changes 

 take place (J. K.) : Along a definite line on the surface of 

 the embryo, marking the region of the back, the outer layer 

 becomes thickened ; the edges of the thickened band so 

 produced rise up on either side, so as to give rise to a 

 median groove between them ; and then, overarching and 

 closing over the groove, convert it into a tube. This tube 

 is called the neural tube, because it gives rise to the 

 central nervous system. In the region of the head it 

 expands ; and from its walls, by the growth and differentia- 

 tion of the cells, there is formed — in the region of the head, 

 the brain, and along the back, the spinal cord. Imme- 

 diately beneath it there is formed a rod of cells, derived 

 from the inner layer. This rod, which is called the noto- 

 chord, is the primitive axial support of the body. Around 

 it eventually is formed the vertebral column, the arches 

 of the vertebrae embracing and protecting the spinal cord. 



Meanwhile there has appeared between the two primitive 

 body-layers a third or middle layer.* The cells of which 

 it is composed arise from the inner layer, or from the lips 

 of the primitive cup when the outer and inner layer pass 

 the one into the other. This middle layer at first forms 

 a more or less continuous sheet of cells between the inner 

 and the outer layers. But ere long it splits into two 

 sheets, of which one remains adherent to the inner layer 

 and one to the outer layer. The former becomes the 

 muscular part of the intestinal or digestive tube, the latter 

 the lining of the body-wall. The space between the two is 

 known as the body-cavity. Beneath the throat the heart 

 is fashioned out of this middle layer. 



Very frequently — that is to say, in many animals — the 



* In technical language, the outer layer of cells is called the epiblast, the 

 inner layer the hypoblast, and the mid-layer between them the mesoblast. 



