606 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
has led to the establishment of the great sub-divisions of the Animal 
Kingdom.” 
Autotomy in Crabs.* — M. L. Fredericq, who commenced the study 
of this subject nearly ten years since, has been engaged in making some 
fresh observations. He finds that autotomy, or the defensive reflex 
mutilation, of the limbs is produced in the Crab at the level of the 
groove which corresponds to the fusion of the basipodite with the 
ischiopodite. The surface of rupture is clean, for the vessels and nerve 
are the only soft organs which are torn through at this point. A special 
diaphragm, the obturator membrane, prevents the escape of blood ; the 
nerve and the vessels cross this membrane at the level of a narrow 
orifice, which is situated excentrically. The non-deciduous portion of 
the second joint of the appendage, or basipodite, does not contain any 
muscular fibres ; but it gives attachment at its proximal edge to the 
tendons of four muscles ; the first joint, or coxopodite, affords attachment 
to two, and the fibres of all six are inserted into the epimera. Section 
of the muscles contained in the deciduous portion of the limb and that 
of five of the six attached to the non-deciduous part has no influence on 
autotomy, which is effected with the same facility after as before the 
operation. Section of the long extensor does prevent autotomy, and it 
may, therefore, be called the autotomist muscle. 
Autotomy may be produced in any position of the limbs, except 
forced extension. For autotomy to be effected by the contraction of the 
autotomist muscle the distal deciduous portion of the limb must have a 
point of support, and the epimeral attachment must also be fixed ; in 
some cases the weight of the animal is sufficient for this. 
Formation of Germ-Layers in Isopoda.| — Dr. J. Playfair M‘Mur- 
rich finds that Jaera albifrons presents conditions favourable for a 
thorough study of a typical centrolecithal segmentation. The egg, 
when passed into the brood-pouch, is of a grass-green colour, and 
enclosed by a single envelope, the chorion ; it is of an oval shape, and there 
is in the centre a stellate mass of protoplasm containing the nucleus, 
while a thin layer of peripheral protoplasm encloses the egg. In 
ovarian eggs which are about half-grown, a delicate protoplasmic network 
extends between the peripheral and central protoplasms. As a result 
of the early cleavages there is produced at one extremity of the egg a 
circle of four nuclei, and at the other a circle of three nuclei surrounding 
a fourth. The last is the ancestor of the endoderm cells. 
A little later it may be seen that the egg is a syncytium, and it is 
probable that, so long as the nuclei are below the surface, a syncytium 
exists, but as they, with their protoplasm, begin to separate from the 
yolk, they become distinct. 
At about the 128-stage there is a concentration of both mesoderm and 
ectoderm cells towards the ventral surface of the egg. Still later the 
mesoderm cells form a plug of cellsVliich projects down a short distance 
into the yolk, forming the so-called blastodisc of the Crustacean egg. In 
some cases a semicircle of ectodermal cells may be seen surrounding the 
front margin of the blastodisc ; each cell gives rise by division to a 
* Arch, de Biol., xii. (1892) pp. 1G9-97 (6 figs.). 
t Zool. Anzeig., xv. (1892) pp 271-5 (2 figs.). 
