ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
343 
Embryology of Isopoda.* — Dr. J. Nusbaum gives a preliminary 
account of the results of his studies on the development of some marine 
Isopods taken at Concarneau. 
Germinal Layers and Digestive Tract of Ligia oceanica. — In the 
earliest stage observed a layer of very finely granular protoplasm was 
seen at one pole of the egg ; this occupied about a third of the periphery 
of the egg, and the whole of the rest was filled up with nutrient yolk. 
Only two oval, fine nuclei, which contained a number of chromatin 
granules, were observed in this protoplasmic layer ; they may be 
regarded as the first products of the segmentation nucleus. In the next 
stage of development the layer extends itself more or less regularly over 
the whole periphery of the egg ; the nuclei multiply and gradually 
extend through the layer. These blastoderm-nuclei elongate consider- 
ably before division ; they appear to move about in amoeboid fashion, as 
they are seen in sections to be provided with pseudopodia ; these seem 
to be the objects which, in Porcellio scaber , Reinhard took for cells. 
In the next stage of development a special layer of protoplasm 
becomes differentiated around each blastoderm-nucleus, and in this way 
a layer of blastoderm-cells is formed which becomes closely packed at 
that pole which corresponds to the futuie ventral surface and the hinder 
part of the embryo ; this is the first sign of the germ-stripe. Somewhat 
later this spot becomes triangular in form and exhibits some signs of 
differentiation ; the rest of the egg is formed of much-flattened cells, 
which are widely separated from one another ; the blastodermal layer is 
several cells thick at various points, and some of the cells separate off 
and wander in the yolk. 
The epithelium of the mid-gut and of the “ hepatic ” tubes is formed 
from two anterior aggregations of endodermal cells, just as in Oniscus. 
The salivary glands are evaginations of the stomodaeum, and not of 
endodermal origin, as the author thought when describing Oniscus. It 
is important to note that the mesoderm arises from paired rudiments, as 
is the typical case in other Enterocoelia. 
The Germ-stripe and the Extremities. — The author recognizes a stage 
corresponding to that of the Nauplius, and in the germ-stripes of this 
stage there are paired optic lobes and paired rudiments of the endoderm. 
The cells of that part of the germ-stripe that lies behind the third pair 
of extremities are regularly and segmentaily arranged ; at the hindermost, 
somewhat thickened and broader end there are some rows of closely 
packed and very regular larger cells, from which new segments are given 
off anteriorly. This segment-forming zone lies in front of what will 
be the anus ; this last corresponds to the hinder part of the blastopore. 
At a later stage there may be observed two optic lobes, two pairs of 
antennae, a pair of mandibles, two pairs of maxillae, and a pair of 
maxillipeds. The last and antepenultimate have two branches, while 
all the extremities of the mid- and hind-body are two-branched. In those 
of the mid-body the outer branch disappears later on. The cephalic 
nervous system is formed from two pairs of ectodermal thickenings 
which lie internally to the antennae, while internally to all the other 
appendages are the rudiments of the ganglia of the ventral chain. 
Externally to all but the four foremost pairs of appendages there are 
* Biol. Centralbl., xi. (1891) pp. 42 9. 
