332 
E. AV. MACBIJIDE. 
process as far as I can follow him in some such manner as 
this : Starting with a supposed holoblastic egg which had a 
larval envelope, the mass of cells destined to form the embryo 
does not develop with equal rapidity to the surrounding 
envelope, and consequently becomes detached from it every- 
whei*e except at one spot, where there is a stalk of connection 
between envelope and embryo. The amniotic cavity between 
true amnion and embryo was originally a water cushion 
developing within the ectoderm to form a protection for the 
embryo, which subsequently derived a more complete pro- 
tection by the method of detachment just described. Along 
the stalk of connection between embryo and vesicle the 
bladder subsequently grew, and so the allantois was formed. 
From this process the simple folding process seen in the 
formation of the amnion in Reptilia is supposed to have 
arisen as a secondary modification. Now an amnion has been 
developed in Insecta, and Hubrecht will find it hard to 
convince any specialist in Ai’thi'opoda that it has come about 
otherwise than as a modification of the folding' of the germ- 
disc seen in its incipient stages in Myriapoda. But why, if a 
protective fold has developed in the Arthropodan egg, should 
there be any difficulty about its development in the Verte- 
brate egg ? Suppose we apply in both cases the same 
hypothesis. The folding of the germ-disc in a Myriapod egg 
is due to its great length and the consequent impossibility of 
its expansion within the egg-shell. In the insect egg the 
germ-disc is proportionately shorter, but the iidierited habit 
of folding' has persisted and has led to the formation of the 
amniotic fold. 
Now ill the Amphibian egg no expansion of the embryo 
takes place till it has escaped from the egg membrane, but 
when the Reptilian ancestor took more completelj^ to life on 
land a longer retention of the embryo within the egg-shell 
and a greater supply of yolk would become a necessity. 
Hence at a certain stage in the development of land animals 
there arose a necessity of bending the head inwards into the 
yolk-sac so as to give room. The most flexible part of the 
