i6 



The ear is formed by a pit, the auditory pit, and like 

 the lens of the eye is an involution of the epiblast, or outer 

 layer of cells, also, as in the eye this involution or pit closes 

 over, but unlike the eye, there is no corresponding involution 

 of the brain to meet this involution of the epiblast. 



On examining the embryo at the third day, great 

 advance in development is seen. 



The blastodermic area is greater, the embryo now lies 

 on its left side, the head has become flexed, and the single 

 tube of the brain has become differentiated into three cavities, 

 the fore, mid, and hind brain. 



The basal pits appear, and as in the eye and ear are 

 formed by a cupping of the epiblast, but this cupping unlike 

 that in the eye or ear never closes. 



The heart is now seen to have receded into the thorax, 

 the throat closes, and a distinct neck appears. 



The single cavity of the heart becomes divided by a 

 partition, and thus two cavities are formed. The gut which 

 is formed by the lower layer of cells or hypoblast is still 

 open in its mid portion, but at the upper and lower end the 

 hypoblast has closed over in front and so completed the tube, 

 the extremities at each end being blind cul de sacs. 



A diverticulum from the upper end of the gut or 

 oesophagus forms the lung on either side. 



Lower down, diverticula from the oesophagus form the 

 liver, pancreas, and spleen. 



The liver has two lobes, each lobe being developed from 

 a separate diverticulum. 



At the lower part of the thorax, the gut expands, and 

 forms the stomach. 



The protovertebrse which we have already noticed on 

 either side of the neural canal become now differentiated, 

 the outer part of each protovertebrae becomes the muscle 

 plate, and from this all the muscles of the back, and finally 

 of the body, become developed ; and the inner part of the 

 protovertebrae fuses with the notochord to form the body of 

 the vertebrae proper, and the spinal nerves. 



