1887] | Embryology. 673 
The latest paper to be recorded is that of Schimkewitsch,' on 
the development of several species of spiders, the main features 
of which were outlined ina preliminary paper in the Zoologischer 
Anzeiger for 1884. After describing the envelopes of the egg 
and the composition of the yelk (of which he recognizes three 
kinds), he proceeds to the segmentation. In this he is inclined 
to follow Ludwig rather than other observers in the recognition 
of a central segmentation and a migration of some of the cells 
to the surface to form the blastoderm, while others remain be- 
hind in the yelk, where, in the shape of polynuclear yelk-masses, 
they represent and finally result in the endoderm. After the 
formation of the blastoderm, three processes occur nearly simul- 
taneously,—the breaking down of the yelk-pyramids, the con- 
centration of the primary ectoderm, and the formation of the 
mesoderm, these being individual variations in the species 
studied. The concentration of the primary ectoderm consists 
in a flattening and consequent expansion: of the cells on one — 
side of the yelk, while on the other they become thicker and 
more cylindrical, thus giving rise to the germinal area. Although 
Schimkewitsch appears to be unaware of this fact, this process is 
paralleled in many Arthropods, and was commented upon at 
some length by Mayer (¥enaische Zeitschrift, xi., 1877). Ac- 
cording to Schimkewitsch, the mesoderm arises, in some species, 
from the blastoderm, in others by budding from the entoderm. 
His figures, however, all seem reconcilable. with the view that 
they all arise from the blastoderm. The primitive cumulus is, 
according to Schimkewitsch, the anal lobe, while the “ white 
spot” of the “ comet-stage” forms the cephalic lobes,—conclu- 
sions somewhat at variance with those of Morin? and Locy, the 
latter reversing the ends of the embryo. 
The account of the external development adds but little to 
our previous knowledge, the principal points being that the 
mandibular segment is budded from the cephalic one, that Crone- 
berg’s antennz are the rudiments of the upper lip, and that no 
appendages are developed at any stage upon the first abdominal 
segment, ‘ 
~ In the internal development there are more points to be 
noticed. The splitting of the mesoderm begins in the fifth seg- 
ment of the body. At first the resulting coelomatic cavities are 
distinct, but soon they run together in the thoracic segments. 
At about this stage begins the formation of a secondary ento- 
derm, composed of cells budded from the polynuclear yelk- 
Masses, and taking a peripheral position in the yelk. These, 
thinks Schimkewitsch, may possibly be the yelk-mesoderm of 
Balfour; but our author does not make it clear how they then 
_ Pass through the splanchnopleure and take a position in the body 
* Archives de Biologie, vi. pp. 515-584, pls. xviii.—xxiii., 1887. 
° Of Am. Nat:, xxi. p. 294, 1887. i Ba + eee 
