108 



THE EARLY DEVELOPMENT OF MAMMALS. 



end of the embryo becomes so large as to rise up from the general surface of the 

 ovum and then to project forward. A very little later a similar process occurs 

 at the caudal end, and the whole body of the embryo rises now above the yolk, 

 and the further growth results not only in a greater protrusion of the head for- 



Op.L. 



To. 



epen. 



EC. 



bas.g. 



FIG. 50. TRANSVERSE SECTION OF AN EMBRYO CATFISH (AMIURUS) ; SERIES 25, SECTION 43. 

 Ac, Aorta, bas.g, Basal ganglion of mid-brain. EC, Ectoderm, epen, Ependymal layer of mid-brain, it, 

 Cavity of mid-brain. Z, Lens. Mk, Meckel's cartilage. N.op, Optic nerve. Op.L, Optic lobe. Per. 

 cat, Pericardial ccelom. PA, Pharynx, pig, Pigment layer of the eye. R, Retina. To, Torus. Trab, 

 Trabecula cranii. x, Undetermined organ. Yk, Yolk. X 4 diams. 



ward and of the tail backward, but also of the body sideways, so that now the 

 embryo appears to have a constricted connection with the rest of the ovum. The 

 general character of the process may be readily understood by comparison of the 

 three diagrammatic cross-sections (Figs. 34, 3 1 , A , and 3 1 , B], and also of the three 



