Germinal Membrane of a Dog
Portion of the germinal membrane, with rudiments of the embryo, from the ovum of a dog. The primitive groove, A, is not yet closed, and at its upper or cephalic end presents three dilatations, B, which correspond to the three divisions or vesicles of the brain. At its lower extremity the groove presents a lancet-shaped dilation (sinus rhomboidalis) C. The margins of the groove consist of clear pellucid nerve substance. Along the bottom of the groove is observed a faint streak, which is probably the chorda dorsalis. D. Vertebral plates.