424 AN INTRODUCTION TO ZOOLOGY 



The cells of the muscle plate elongate to form characteristic " myo 



EZ-m G?Y 

 cewf/. & MM \ QpMJffff &M. 



-n-k rr-r-. ^-> ?-> jrsf -j-r ^ i ^-^-'-7 77 '7? *?~tt ' \ 



^/tW\ 



erf) 



FIG. 148. 'Chick embryo with thirty-five pairs of somites (72 hours). 



From Lillie. 



a.a., 1,2,3,4, first, second, third, and fourth aortic arches ; Ar., artery ; A.V., vitelline artery ; 

 cerv. Fl., cervical flexure ; cr.Fl., cranial flexure ; D.C., duct of Cuvier ; D.V., ductus venosus ; 

 Ep., epiphysis ; Gn.V., ganglion of trigeminus ; Isth., isthmus ; Jug.ex., external jugular vein ; 

 Md., mandibular arch ; M.M., maxillo-mandibular branch of the trigeminus ; olf.P., olfactory pit ; 

 Ophth.. ophthalmic branch of the trigeminus ; Ot., otocyst ; V., vein ; W.B., wing bud ; V.c.p., 

 posterior cardinal vein ; V.umb., umbilical vein; V.V., vitelline vein ; V.V.p., posterior vitelline 

 vein. 



blasts," that give rise later to the muscles. They are responsible 

 for the whole of the voluntary musculature of the adult, while the 



