358 WHEELER. [Vot. III. 
ably due to the fact that the embryonic envelopes are to be 
absorbed. As these membranes consist of assimilable matter, 
it is obviously an advantage to the embryo to be able to add 
them to the stock of food represented by the yolk. The simplest 
conceivable method of effecting the resolution of the envelopes 
into food material, considering their position when fully devel- 
oped, would, of course, be to engulf them in the yolk, where, 
under the influence of the yolk cells, metabolism is being 
actively carried on. There are two methods of inclosing the 
membranes in the yolk. According to one, they might undergo 
dissolution zz situ, according to the other, they might be 
brought together in a mass and swallowed up by the yolk some- 
where in the median dorsal line. Obviously the latter method 
is the more advantageous, as the body walls, continually grow- 
ing towards the median dorsal line, might be impeded in their 
advance if the membranes were absorbed at all points on the 
surface of the yolk. Probably the inconvenience which would 
thus result from a diffuse absorption accounts for the fact that 
it does not occur, though a modification of the method occurs 
in Doryphora, where the serosa is absorbed very late in develop- 
ment after the larva has secreted its second cuticle and is almost 
ready to leave the egg. 
Given a thickening, somewhat flattened mass of cells, des- 
tined to be swallowed up in the yolk, and it is most natural to 
suppose that the organ, in passing into the yolk, would become 
cup-shaped as in A/azta, or form a thick-walled tube, if the organ 
extended the full length of the dorsum, as in Hydrophilus. In 
either case the outer ends would be made to converge, by the 
lateral pressure of the yolk and the sinking of the median por- 
tion of the organ, and we should get a closed tube or sack. This 
would not, of course, hold true of an organ formed like the 
amniotic dorsal organ of Doryphora, for the reason that in this 
case the decomposition begins as soon as the organ is formed, 
and not after it has passed into the yolk, as is the case with 
the serosal dorsal organs of other forms (//ydrophilus, Blatta, 
Neophalax). Hence the cavity of the Hexapod dorsal organ 
would resemble the cavity or so-called micropyle of the Isopod 
dorsal organ, though the two cavities would not be homologous. 
