THE EARLY DEVELOPMENT OF MAMMALS. 



N.fac.ac. Ot. O. N.gl.ph. N.vag. Dor.com. 



Hi. 



So. ant 



Op. ves. 



G.cl. Md.st. Hyo.st. 



Prcd. 



FIG. 48. SQUALUS ACANTHIAS, 9.0 MM. SERIES 1495. RECONSTRUCTION TO SHOW THE HEAD-CAVITIES. 



BY R. E. SCAMMON. 



Ao, Dorsal aorta. Dor. com, Dorsal ganglionic commissure. G.cl, First gill-cleft; two others are open and 

 marked by similar shading. Ht, Heart. Hyo. st, Stalk of mesoderm connecting the hyoid cavity with 

 the pericardium. Md. st, Stalk of mesoderm connecting the mandibular cavity with the pericardium. 

 Nch, Notochord. N. }ac. ac, Facial-acoustic nerve trunk. 2V. gl. ph, Glossopharyngeal nerve. N. vag, 

 Vagus nerve. TV. ar. tr, Temporary ganglionic mass (Dohrn's "Urtrochlearis"). O, Orifice of the 

 otocyst. Op.ves, Optic vesicle. Ot, Otocyst. Prcd, Pericardium. So. ant, Anterior head-cavity, a 

 derivative of the premandibular cavity, and lacking in most animals. So.hyo, Hyoid head-cavity. 

 So.mand, Mandibular head-cavity. So.prem, Premandibular headcavity X 30 diams. 



