2 24 Vertebrate Embryology 



One of the most important and character- 

 istic events of this day is the appearance of 

 the appendages, the wings and legs. They 

 are formed as local swellings or thickenings 

 of the Wolffian ridge. Each limb consists of 

 a core of compact mesoblast covered with a 

 layer of ectoblast. The swellings that are to 

 form the wings appear just back of the region 

 of the heart, while the legs are formed just in 

 front of the tail (Fig. 72, f.l and A./). The 

 limb buds are, at first, conical or triangular in 

 outline, but they soon begin to differentiate, so 

 that, by the end of this day, it may be possi- 

 ble to distinguish between the wings and legs 

 by their shape as well as by their position ; 

 the wings being comparatively long and nar- 

 row, while the legs are short and broad. 



The head. — The cerebral hemispheres (Fig. 

 51, VH^ are becoming very large compared 

 with the thalamencephalon, Vi, from which they 

 sprang ; and the separation of the cerebellum 

 from the medulla is becoming more marked. 

 The mid-brain is now relatively larger than at 

 any other time, and forms the large rounded 

 protuberance at the angle of the cranial 

 flexure (Fig. 72, m.br^. The eyes are enor- 

 mously large, compared with the size of the 



