DEVELOPMENT OF THE CHICK THIRD PERIOD. 



325 



size. The germinal membrane now extends over the whole 

 of the yolk ; or the mucous layer of this part has almost en- 

 tirely grown around, and so 

 given origin to a sac-like co- 

 vering, the vitellary sac 

 (vitelliculum, or vitellicle, 

 Owen), which encloses the 

 yolk ; the vascular layer has 

 grown around nearly two- 

 thirds of the yolk. The si- 

 nus terminalis of this layer 

 is now a mere seam in the 

 periphery of the area vascu- 

 losa, and in the course of the 

 next few days disappears en- 

 tirely ; the veins, and then 

 the arteries of the vascular 

 layer of the vitellary mem- 

 brane, disappear somewhat 

 later. On the other hand, 

 the allantois is growing with 

 great rapidity, and, on the 

 sixth day, forms a pretty large flattened bladder (fig. 349), 

 which, however, in the course of the seventh day, acquires 

 nearly twice its former size, and inclines so much to the 

 right side, that with the amnion, it covers the embryo com- 

 pletely, and comes in contact superiorly by means of its most 

 vascular side with the serous envelope, which is consequently 

 now completely separated from the amnion, to the formation 

 of which it had in the first instance contributed. After the 

 rupture of the vitellary membrane, all that remains of the al- 

 bumen collects at the sharp end of the egg, and is now much 

 more consistent ; the yolk, on the contrary, has become 

 much thinner and more diffluent, and the number of its glo- 

 bules has very greatly diminished ; the embryo lies more to- 

 wards the blunt pole of the egg, and on the sixth day, after 

 breaking open the shell, the first appearance of motion is 

 observed in slight twitchings of the extremities. 



[ 494. The most remarkable metamorphoses of the indi- 

 vidual organs on the sixth and seventh days are the following : 

 the spinous processes are now formed on the vertebral arches ; 



Fig. 349. Embryo of the fowl 

 with the allantois, a, already of 

 great size, and depressed or flat- 

 tened, the umbilical vessels, b, 

 branching over it ; c, external ear, 

 indicated by a depression ; d, cere- 

 bellum ; e, corpora quadrigeinina ; 

 /, hemispheres. 



