50 



THE EARLY DEVELOPMENT OF MAMMALS. 



Op.L. 



Ao. 

 Ph. 



Mk. 



brate' series, an increasing tendency to give the embryo prominence and to differ- 

 entiate it more decisively from the embryonic appendages. This becomes so 

 marked in the higher vertebrates that we speak of the growth of the embryo 

 T epen . almost as a separate thing from 



the growth of the appendages. 



The embryo is developed from 

 the axial portion of the embryonic 

 shield, the position of which is 

 marked by the primitive streak 

 (Fig. 14, p.s). In the territory 

 around the embryo are developed 

 the first blood-vessels, hence it is 

 termed the area vasculosa (see 

 Per.cce. P a g e 66). About the time that 

 the blood-vessels begin to appear, 

 the separation of the embryo from 

 the shield commences, and the 

 extra-embryonic portion of the 

 shield remains as part of the 

 blastodermic vesicle, or yolk-sac. 

 This separation is due wholly to 

 the growth of the embryo.* 

 The process is illustrated by the 

 diagrams (Fig. 16), in which for 

 greater clearness the blastodermic 

 vesicle is represented filled with 

 yolk, as it is in the Sauropsida. 

 Soon after the blood-vessels ap- 

 FIG. 17. TRANSVERSE SECTION OF AN EMBRYO CATFISH pear, the head of the embryo has 



(AMIURUS); SERIES 25, SECTION 43 . grown so much that it not only 



Ao, Aorta, bas.g, Basal ganglion of mid-brain. EC, Ectoderm. Hn +fc f f tVi 



epen, Ependymal layer of mid-brain, it, Cavity of mid-brain. 



L, Lens. Mk, Meckel's cartilage. N.op, Optic nerve, shield, but projects forward (Fig. 

 Op. L, Optic lobe. Per.cce, Pericardial ccelom. PA, Pharynx. 16, A, K). Later the caudal end 

 ^Pigment layer of the eye *, Retina. To, Torus. Trab, becomes free in the same 

 Trabecula cranu. x, Undetermined organ. Yk, Yolk. 



X4odiams. (Fig. 1 6, B, C). Cross sections 



show a similar expansion of the 



embryo laterally (compare the three diagrams, Figs. 29, 45, A, and 45, B). ' Hence, 

 though the connection between the embryo and the blastodermic vesicle may remain 



* The separation of the embryo from the rest of the ovum has long been described as a process of the folding 

 the germ layers on the under side of the body. The traditional perpetuation of this erroneous description 

 s regrettable, for the separation of the embryo is really due to the expansion of the embryo, and in no sense to the 

 constriction of its connection with the yolk. 



Ec.\ 



