ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
457 
lobes, from which the eyes and brain are, later on, formed. These 
lobes converge in the direction of the ventral median line, touch, and 
give rise to a considerable thickening, which is the rudiment of the 
labrum. The lobes and the disc together give rise to a germinal stripe, 
which corresponds to the ventral side of the embryo. The changes in 
the germinal disc are next described, and this is followed by an account 
of the changes in the cephalic lobes, and the formation of the nervous 
system. 
In the Nauplius-stage the egg is completely invested by the continuous 
blastodermal layer. The rudiment of the eye is now separated on either 
side from the optic ganglion, and later on constricts off from the ecto- 
derm a complex of ectodermal cells, which gives rise to the median 
elements of the eye. The ganglionic rudiments cease to be solid cell- 
aggregates, for each becomes hollowed as the ganglion-cells and dotted 
substance begin to be differentiated. This dotted substance appears to 
be the result of a direct conversion of some of the true ganglion-cells. 
The ganglion-cell elongates, becomes spindle-shaped, and finally breaks 
up into separate fibrils. Though the three pairs of appendages charac- 
teristic of the Nauplius-stage are somewhat developed, they are not yet 
jointed. The divisions of the digestive tract become apparent. The 
abdomen appears as an elongated process, which runs parallel to the 
ventral surface ; the rectum is continued into it in the form of a 
cylindrical tubule, the blind end of which extends into the mass of 
amoeboid cells, which are here arranged in two rows. The mesodermal 
cells become connected with those which have been given off from the 
germinal disc and the lateral ectodermal thickenings, and give rise to 
two mesodermal bands on either side of the median line. These bands 
become metamerically jointed, and, later on, give rise to the body- 
cavity, splanchnopleure, and somatopleure. 
The rudiment of the heart appears in the form of a rounded solid 
mass of mesodermal cells, and lies between the thorax and abdomen ; 
its cells have a coarsely granular protoplasm, and stain very strongly 
with borax-carmine. Later on, the peripheral cells elongate and form a 
unilaminate membrane, when the heart appears as a completely closed 
cavity. In its interior there are a few mesodermal cells, and some blood- 
corpuscles. It now begins to beat, but the regular rhythmical contrac- 
tions are seen only in the inner mesodermal membrane, while the outer 
ectodermal wall, which has no muscular elements, playS only a passive 
part. Later on the mesodermal cardiac membrane is further differen- 
tiated, and in the zoea-stage the heart has the form of an elongated 
spindle-shaped tubule, the wall of which is everywhere delicate. 
The first rudiments of the segmental organs are found in embryos 
shortly before the zoea-stage ; they appear as a paired evagination of the 
somatopleure. They are ventral in position. The elongated lens-like 
cells of the somatopleure in their neighbourhood become cubical and 
cylindrical. Each cell of the evaginated wall has in its outer part a 
nucleus and some protoplasm, while the rest is free of protoplasm and 
highly cuticularized. The distal end of the evagination passes into a 
blind tubule, which elongates and soon forms a canal which, after 
several coils, ends blindly beneath the skin. Here the ectoderm 
becomes invaginated and forms a short tubule, the blind end of which 
1890. 2 k 
