1887] Embryology. 491 
Phascolarctos from the uterus I found a chorion like that of 
As in Monotremata, a coagulable, and, when coagulated, 
deeply-staining fluid makes its appearance between the vitelline 
membrane and ovum (blastoderm). 
“The shell-membrane persists until the developing ovum be- 
comes fixed to the. walls of the uterus, after which it disap- 
pears.” 
The paper then compares the egg-membranes just described 
-with those of Placentalia and those of Vertebrata generally. 
va 
he ye This unenclosed area in front of the primitive streak 
probably includes a region where the hypoblast (yelk) has sec- 
ondarily broken through the epiblast. existence of such a 
- marked out by the growing epiblast and the posterior lip of the 
blastopore, before the closing of the primitive-streak region, or 
