II.] THE embryonic: appendages. 33 



As a, result of these several changes, a great iucrease iu 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 ou the 

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

 serous envelope. 



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



In X the splanchnopleure has completely invested the yolk- 

 sac, but at the lower pole of the yolk is still continuous with 

 that peripheral remnant of the somatopleuie now called the 

 serous membrane. In other words, the cleavage of the mesoblast 

 has been carried all round the yolk (ys) except just at the lower 

 pole. 



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-sac (ys) is 

 therefore quite loose in the pleuroperitoneal cavity, being con- 

 nected only with the alimentary canal {a') by a solid pedicle. 



Lastly, in JV the yolk-sac (ys) is shewn being withdrawn into 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 i/s 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 juxta- 

 position. 



In a vertical longitudinal section carried through the 

 middle Hne, we may recognize the following parts (Fig. 

 9, A, or on a larger scale Fig. 10, which also shews details 

 which need not be considered now). Beginning at what 

 will become the posterior extremity of the embryo (the 

 left-hand side of the figure in each case), and following the 

 surface of the blastoderm forwards (to the right in the 

 F.&B. 3 



