ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
469 
and irregularities occur in the keel and egg-nauplius stages. Degenerat- 
ing nuclei occur in the fully-formed egg-nauplius, and are most noticeable 
in the region of the stomodaeum and optic discs. The author believes 
that the fragmentation and dissolution of cells is a common phenomenon 
among the Crustacea and other Arthropods. 
Development of Daphnia from the Summer-egg.* — Mr. J. Lebedin- 
sky has made a study of Daphnia similis , the summer-egg of which is 
quite spherical and 0 * 125 mm. in diameter ; it is invested in a chorion 
and a vitelline membrane ; the nutrient yolk is concentrically arranged, 
is green or blue in colour, and makes the egg quite opaque. In each 
egg there is always a large excentrically placed fat-sphere, around 
which smaller ones are grouped : the protoplasm is amoeboid, has a 
lobate zone, and takes up the yolk. 
Segmentation is superficial ; the descendants of the amoeboid cell 
multiply by division and creep from the centre to the periphery of the 
egg, only a few remaining in its interior ; others give rise to plasmodia. 
In time a continuous blastodermal layer is formed, the cells of which 
are all of the same size and form. Some of the cells become high and 
cylindrical and form the elongated germ-stripe. The embryo is now 
bilaterally symmetrical, but has still a spherical form. The blastopore 
is a slight depression below which are a few amoeboid cells which slowly 
sink into the yolk. These cells form the meso-endoderm which becomes 
differentiated into separate, independent layers. The endoderm forms a 
solid cord in which cavities appear later on. All the endodermal cells 
do not form part of the mid-gut, as some extend over the nutrient yolk, 
and form two large provisional liver-sacs. 
The shell-gland is formed as a paired mass of mesodermal cells, 
which are clearly distinguished from their neighbours by their structure 
and size ; the heart is at first an aggregate of mesodermal cells ; later 
on the peripheral cells form a unilaminate pericardium. No special 
genital cells are present in the early stages of cleavage, and the 
rudiments of the gonads are not to be recognized in the nauplius 
stage. 
Vermes, 
a. Annelida. 
Innervation of Proboscis of Glycera.f — M. E. Jourdan finds that in 
the muscular sheath of the proboscis of Glijcera there are eighteen nerve- 
fibres ; these end in a collar which is arranged around the opening of the 
proboscis, and which contains numerous nerve-cells ; it may be called a 
proboscidial nerve-ring. The fibres penetrate into the epithelial layer, 
and are distributed in the very curious papillae which are found on the 
surface of the organ. At the extremity of the proboscis the nerve- 
elements enter into relation with an epithelial pad, which is set 
like a crown behind the hooks. The papillae of the proboscis are of two 
types; some are cylindroconical, and others are irregularly spherical 
and analogous to fungiform papillae. The investing cuticle is very 
delicate and perforated at a point which corresponds to the tip of these 
organs. The body of each papilla is formed of a pigmented protoplasm ; 
* Zool. Anzeig., xiv. (1891) pp. 149-52. 
t Comptes Rendus, cxii. (1891) pp. 882-3. 
