AVES. 



189 



in), and then spreading outwards as before over the yolk. The point at which it 

 unites with the somatopleure, marking the extreme limit of the cleavage of the 

 mesoblast, is now much nearer the lower pole of the diminished yolk. 



As a result of these several changes, a great increase in the dotted space has taken 

 place. It is now possible to pass from the actual peritoneal cavity within the body, 

 on the one hand round a great portion of the circumference of the yolk, and on the 

 other hand above the amnion a, in the space between it and the serous envelope. 



Into this space the allantois is seen spreading in K at al. 



In L the splanchnopleure has completely invested the yolk-sack, but at the lower 

 pole of the yolk is still continuous with that peripheral remnant of the somatopleure 

 now called the serous membrane. In other words, cleavage of the mesoblast has 

 been carried all round the yolk (ys) except at the very lower pole. 



vl 



--H-PP 



In M the cleavage has been carried through the pole itself; the peripheral portion 

 of the splanchnopleure forms a complete investment of the yolk quite unconnected with 

 the peripheral portion of the somatopleure, which now exists as a continuous membrane 

 lining the interior of the shell. The yolk-sack (ys) is therefore quite loose in the 

 pleuroperitoneal cavity, being connected only with the alimentary canal (a') by a 

 solid pedicle. 



Lastly, in N the yolk-sack (ys) is shewn being withdrawn into the cavity of the 

 body of the embryo. The allantois is as before, for the sake of simplicity, omitted ; 

 its pedicle would of course lie by the side of ys in the somatic stalk marked by the 

 usual dotted shading. 



It may be repeated that the above are diagrams, the various spaces being shewn 

 distended, whereas in many of them in the actual egg the walls have collapsed, and 

 are in near juxtaposition. 



little distance behind the tail, and at some little distance from 

 the side (fig. 121 C, D, E, F, and 116, am}. In this way the 



