ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
783 
It fed on water-melons. The author laments that the manufactory, to 
which the wood with its far-travelled inmates came, now employs a 
machine for cutting up the blocks, hence he will receive no more “ blinde 
Passagiere.” 
Development of Mites.* — Herr J. Wagner has found Ixodes a 
suitable object for the study of development, as the process is pretty 
slow and the eggs do not require much care. The segmentation of the 
egg differs from that of other mites hitherto observed in that the 
cleavage process belongs to the type of partial interlecithal segmenta- 
tion. Later on the cells pass to the surface and form the blastoderm. 
Some of the cells are distinguished by their large nuclei, which ordi- 
narily do not lie parallel to the surface of the egg; these nuclei 
are stained by carmine more feebly than those of the ordinary 
blastoderm-cells, and they contain one or two distinct nucleoli which 
are not present in the nuclei of the other cells. These cells retreat 
from the surface to the interior of the yolk, and the blastoderm-cells 
which surround them close together over them ; they form the so-called 
yolk-cells. Some of them become heaped into a mass which forms 
part of the endoderm. 
At the side of this endodermal mass the mesoderm cells become 
apparent ; on either side of the mass there is in the early stages of the 
development of the mesoderm a superficial groove-like depression, at 
the base of which there is a steady immigration of cells. These de- 
pressions appear to correspond with the lateral margins of the germ- 
stripes of Insects. 
During the cleavage of the egg no division of the yolk is to be 
observed. When, however, the appendages are being formed, the yolk 
loses its homogeneous appearance, and breaks up into several polygonal 
pieces, separated from one another by clefts. Later on this appearance 
is still more marked. 
The germ-stripe of Mites, as of Spiders, occupies, during the stage 
of the appearance of the appendages, the greater part of the circum- 
ference of the egg ; in this stage it consists of two ectodermal ridges 
which are separated by a band of flat cells, and meet at the ends of the 
elliptical egg. Looked at from the side, the extremities are seen to be 
very sharply marked off. In addition to the ordinary and characteristic 
three pairs of legs a fourth pair appears ; this, after showing signs of 
undoubted segmentation, begins to suddenly retrograde shortly before 
the extrusion of the larva, so that soon there is no sign of the pair left. 
Sections, however, of the larva show that there is an aggregation of 
cells which form the remnant of the fourth pair of legs. It is from 
this mass of cells that the fourth pair of limbs appear to be developed, 
when the larva passes into the nymph. 
The author concludes with a short account of the segmentation of 
the abdomen and the extremities of the head. Behind the appendages 
there are five or six mesodermal groups ; the mesoderm is best developed 
in the second, third, and fourth segments, in each of which a slight 
protuberance makes its appearance ; these are undoubtedly homologous 
with the abdominal extremities of Spiders. There are no rudiments of 
any extremities in front of the chelicerse, but between them and the 
* Zool. Anzeig., xv. (1891) pp. 316-20. 
