426 ARTHUR ROBINSON. 
in the greater part of its extent and for a considerable period, 
by epiblast alone. I have endeavoured to show that the 
observations which have been brought forward in support of 
such an improbable occurrence are not conclusive, and that 
the facts are capable of a very different interpretation, which 
may be summarised as follows : 
The phenomena observed in the early stages of development 
of the ova of the rabbit, the guinea-pig, the mole, the shrew, 
the bat, and the opossum do not differ essentially from the 
phenomena observable in the ova of all other Vertebrata. 
The blastodermic cavity in the ova of the mammals in 
question is in reality a modified archenteric space which has 
been extended coincidently with the disappearance of the 
solid yolk, for the purpose of maintaining an extensive foetal 
area in close relation with the maternal tissues. 
The whole of the immediate boundary wall of the space in 
question is from the first constituted by the hypoblast, in 
the midst of which it has appeared. 
The only epiblast present in the early stages in the ova of 
the above animals is a small mass which is situated upon one 
pole of hypoblastic mass, which constitutes the greater part 
of each ovum. 
This view appears to me not only more probable than that 
usually held, but also more capable of direct support; and 
although most of the facts and opinions upon which it is 
based have been already referred to, it is advisable, even at the 
expense of a certain amount of repetition, to conclude this 
section with a short summary of the most important. 
1. There seems to be every probability that the Mammalia 
and Sauropsida have been produced from a common ancestral 
stock, whose descendants have diverged along two different 
lines. 
2. It is generally admitted that the ova of this common 
ancestral stock, the “ Protamniota,’? were provided with a 
large amount of food-yolk, and there can be but little doubt 
that all true yolk is to be considered as modified hypoblast. 
3. In the descent from the “ Protamniota” to the Saurop- 
