346 WHEELER. [Vot. III. 
metabolizing the yolk, their outer ends are at the same time 
giving off into the amniotic cavity a less amount of liquid waste 
products. Providing this supposition is true, we should have a 
sufficient reason for the constant closure of the amniotic cavity. 
We have a complete series of finely graduated forms of enve- 
lope formation from the method observed in Calopteryx (Fig. 4) 
to Blatta (Fig. 7). Aphis represents the first step in the tran- 
sition of an enxtoblastic embryo like Calopteryx to the decidedly 
ectoblastic form seen in 4latta. This transition consists in 
leaving more and more of the anterior end of the embryo on 
the surface of the yolk. In Aphzs (Fig. 5) the whole head is 
left outside the invagination; in Doryphora, the head and the 
anterior half of the body. When a portion of the embryo is left 
on the surface, the closure of the amniotic cavity necessitates a 
backward growth of the angle formed by the fore end of the 
head and the abutting serosa (as, Fig. 5) to form a fold which 
unites with a similar fold formed at the opposite end of the 
embryo. In Doryphora, where much of the embryo lies on the 
surface of the yolk, the posterior or caudal fold of the amnion 
and serosa has to grow forward a considerable distance to meet 
the cephalic fold. A/atta has advanced still further than Dory- 
phora. The embryo no longer grows into the yolk, but the 
formation and ultimate closing of the membranes continues. 
At first sight it would seem more natural to suppose that the 
result attained in Blatta was brought about simply by an extru- 
sion of the yolk between the amnion and serosa of such a form 
as Aphis or Calopteryx, but the law of orientation, as explained 
in a preceding paragraph, forbids such an interpretation. The 
head of the Aphis embryo is at the time of the completion of 
the membranes close to the spot before occupied by the caudal 
end of the ventral plate, and after revolution the caudal end of 
the embryo will again be located at this end of the egg. Hence 
the typical ectoblastic originated from the typical entoblastic 
embryo, not by an extrusion of the yolk from between the 
amnion and serosa, but by a gradual weakening of the invagi- 
native process. The weakening, of course, results in more and 
more of the anterior portion of the ventral plate remaining inert, 
though the growth of the membranes to shut off the amniotic 
cavity continues. | 
The peculiar free entoblastic embryo observed in Lepidoptera 
