illae arise from the first fold, the upper jaws being two branches of 

 the fold, the fold itself being the under jaw, while a lozenge- 

 shaped cavity between the fold and its branches becomes the 



Meanwhile, for all the changes in the different organs are going 



■ • 



oping, us well as the separation of the hind-brain into the cerebrel- 

 lnm and nwAulla obUnujata. The digestive cavity is during the third 

 day also, differentiated into the fore-gut and hind-gut, the former 

 farther subdividing into the oesophagus, stomach and duodenum, 

 and the hind-gut into the large intestine and cloaca. The lungs 

 arise as two pocket-like appendages of the alimentary canal im- 

 mediately in front of the stomach; while the liver is originally 

 two diverticula, and the pancreas a single offshoot from the duo- 

 Fourth day. With a decided increase in size by this day, the 

 amnion becomes more distinct, and the allantois is visible. The 

 wings and legs now appear as flattened conical buds arising from 

 the "Wolffian ridge," a low ridge running from the neck to the 

 tail, those forming the wings being scarcely distinguishable from 



The olfactory grooves appear at this time and the partition 

 heretofore existing between the mouth and throat is absorbed and 



The protovertebrae have, by this time, increased in number from 

 thirty to forty. The upper portion (muscle-plate) having previ- 

 ously separated to form the muscles inserted in the skeleton (epi- 

 sketal muscles of Huxley), has left the remainder of each proto- 



enclosing the nerve-canal. On the lower side each protovertebra 

 sends out a similar growth enclosing the notochord. " While the 

 inner portion of each protovertebra is thus extending inwards 

 around both notochord and neural canal, the remaining outer por- 

 tion is undergoing a remarkable change. It becomes divided into 

 an anterior or praeaxial, and a posterior or postaxial segment. 

 The anterior, which is the larger and more transparent of the two, 

 is the rudiment of the spinal ganglion and nerve, while the pos- 



