ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
33 
The formation of the germ-layer, and embryonic envelopes is next 
discussed. The method of germ-layer-formation in Blatta is, at first 
sight, very different from that observed in Doryplwra ; but a study of 
the latter helps to explain the conditions which obtain in the former, 
and makes it probable that the endoderm of Blatta originates in the 
mass of cells found under the area of jiroliferation, the more superficial 
cells of which form the mesoderm. 
The embryonic envelopes and the dorsal organ which are formed soon 
after their rupture, offer a good deal of difficulty. In considering the 
meaning of the amnion and serosa of Hexapoda, we must postulate that 
(1) there are no sufficient reasons for homologizing the embryonic 
envelopes of insects with the homonymous but dissimilar structures in 
Myriopods, Scorpions, and Peripatus ; (P) there is no more than a 
superficial resemblance to speak for a homology between the dorsal 
organs of the Crustacea and the embryonic envelopes of Insects, and 
between the dorsal organs of the former and the homonymous structures 
in the latter; (3) the dorsal organ of insects may be regarded as 
the necessary result of the rupture and absorption of the embryonic 
envelopes, and, consequently, as in no way related to such structures 
as the dorsal organs of Cymothoa, Limulus, and others. There is a 
complete series of finely graduated forms of envelope-formation from 
that seen in Calopteryx to that which obtains in Blatta, Aphis repre- 
senting the first step in the transition of an entoblastic into an ecto- 
blastic embryo. The author concludes that 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 weaken- 
ing of the invaginative process. This weakening 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 term “ dorsal organ ” is applied to the peculiar thick lump of 
cells which results from the concentration on the dorsal yolk of the 
remains of either the amnion or serosa or of both, preparatory to their 
absorption in the yolk. Its presence in Insects is probably due to the 
fact that the embryonic envelopes are to be absorbed. These mem- 
branes might either undergo dissolution in situ, or they might be 
brought together in a mass, and swallowed up by the yolk somewhere in 
the median dorsal line. The latter method is obviously the more 
advantageous as the body-walls, which are continually growing towards 
this dorsal line, might be impeded in their advance, if the membranes 
were absorbed at all points on the surface of the yolk. 
In conclusion, Mr. Wheeler gives an account of the fate of the 
different germ-layers. We have only space to note that the frontal 
ganglion is formed as an unpaired thickening of the dorsal wall of 
the oesophageal ectoderm near the base of the labrum ; the outer neuri- 
lemma is of ectodermal and not of mesodermal origin, for shortly after 
the separation of the nerve-cord from the integumentary ectoderm, it 
sheds from its surface a delicate chitinous cuticle, simultaneously with 
the shedding of the first integumentary cuticle. 
1890. 
D 
