DESCRIPTION OF THE FIGURES. 



DESCRIPTION OF THE FIGURES. 



Figs. 3-7. Natural size; the appendages are represented in section, 

 and are semi-diagrammatic. 



d, Outer embryonic sac or tunic. 

 a, b, c, Absorbing area (yolk pla- 

 centa), consisting of cells 

 (trophoblastic) of the em- 

 bryonic sac, and of the 

 inner (hypoblastic) cells of 

 the yolk sac. 



an,, Section of ring (annulus) sur- 

 rounding absorbing area. 



t.g., Girdle (trophoblastic), formed 

 entirely by a modification 

 of the outer cells of the 

 embryonic sac. 



am. , Amnion — the amnion eventu- 

 ally blends with the allan- 

 tois. 



all., Allantois — the stalk connect- 

 ing the allantois with the 

 embryo is only shown in 

 figs. 1 and 2. 

 all.,i Allantois completely fused 

 with the embryonic sac, 

 which provides a covering 

 for the allantoic villi. 

 all.,' 2 Allantois in connection with 

 the amnion. 

 aU. 3 -all.,* The part of the allantois in 

 connection with the yolk 

 sac — in the fifth to the 

 eighth week embryos this 



part of the allantois carries 

 non-vascular villi (c.v.). 

 all. villi, Allantoic villi, each with 

 a covering derived from 

 the embryonic sac. 



c.v.-, Villi (coelomic) lying between 

 the allantois and yolk sac. 

 These internal villi are 

 naked (not invested like 

 the external villi), and are 

 non-vascular ; they have 

 not hitherto been found in 

 any mammal, and their 

 function is unknown. 



sta. , Part of the umbilical cord 

 within the amnion formed 

 by the junction of the allan- 

 toic and yolk stalks, and 

 containing the main trunks 

 of the allantoic and yolk 

 sac vessels. 

 v., These dots indicate that the 

 greater part of the wall of 

 the j^olk sac is vascular. 

 v., 1 These dots indicate that the 

 allantois is vascular through - 

 out its whole extent. 

 v.,' 2 Allantoic vessels, from which 

 branches pass into the villi. 



y.s., Yolk sac. 



Fig. l.'The hen's egg at the ninth day of incubation. The embryo 

 is connected by a short stalk to the yolk sac, which contains 

 nearly all the food required during the developmental pro- 

 cess. The additional nourishment is provided by the 

 white or albumen which surrounds the yolk. The allantois 

 at an early stage grows out from the intestinal tract, be- 

 comes extremely vascular, and plays the part of a breathing- 

 organ. Numerous vessels are also formed in tbe wall of 

 the yolk sac ; the dots are intended to indicate that the 

 yolk sac and allantois are vascular. The shell is lined with 

 a thin membrane. At one end this membrane is separated 

 from the shell, leaving a space — the air chamber. After 

 Milnes Marshall. 



Fig. 2. Represents a young opossum and its foetal appendages. The 

 wall of the yolk sac (y.s.) is vascidar as far as the circular 

 blood vessel (s.t., sinus terminalis). In the area a, b, c, the 

 yolk sac blends with the outer embryonic sac. Through 



