ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
39 
epithelium, while the inner, which is delicate, is the cell-membrane of 
the egg. The cavity around the germinal vesicle does not appear to be 
an artificial product. A large number of forked chromosomes are to 
be found in the germinal vesicle. The nucleoli disappear when the 
chromosomes give rise to the first directive spindle. As the ovum 
matures, the germinal vesicle migrates to the surface of the egg. The 
first polar globule has about eight chromosomes. The first spindle 
appears to be formed from the remains of the germinal vesicle, and no 
polar rays or centrosome can be seen in it. The first spindle is 
primarily tangential in position, and then radial. When it is radial 
its fibres are attached to the c< 11-membrane of the egg, and to the 
plasmatic network of the egg-cell. The first polar globule is expelled 
as the egg passes from the ovary into the oviduct, or in the uppermost 
portion of the latter. The two directive divisions are true cell-divisions ; 
and the two cells lie between the yolk-membrane and the egg-cell- 
membrane. The second spindle is derived without any period of repose 
from the first, is much smaller than it, and, like it, has no signs of polar 
rays or centrosome. The second cell is not expelled till after fertiliza- 
tion. In its migration the ovarian nucleus, which contains neither 
spheres nor centrosomes, grows very considerably. 
The spermatozoon consists of head, connecting-piece, and tail; in 
the head a Retzius’ spear, a special anterior piece, and an axial filament 
can be made out ; the connecting-piece consists of an axial filament and 
a mantle ; with metallic salts and haematoxylin it is stained a deep 
blackish blue. The tail has an axial filament and mantle, as well as 
an undulating membrane, and a specially formed terminal piece ; the 
membrane is attached on one side only, and not spirally. 
At the point where the spermatozoon enters the egg there is formed 
a funnel-shaped or conical aggregation of ovarian plasm, and the whole 
of the spermatozoon passes into the egg. From the connective piece of 
the spermatozoon an attraction sphere is developed, and from the head 
the spermatic nucleus is formed, while the tail disappears completely. 
This nucleus migrates and exhibits amoeboid movements, while it 
increases considerably in size. 
In the Axolotl there is physiological polyspermy; the subsidiary 
spermatozoa behave exactly like the chief spermatozoon, but they do not 
copulate with the ovarian nucleus. 
Ontogenetic Differentiations of Ectoderm in Necturus.* — Miss 
J. B. Platt has studied the development of the neural crest in Necturm , 
and finds that it presents some differences from what obtains in the 
Bird. The neural crest arises in two, not three, divisions, and each 
contributes directly to the formation of the peripheral nervous system. 
They do not constitute merely a nerve-supporting tissue, but give rise 
to ganglion-cells which “ spin ” motor as well as sensory fibres. Cells 
arising from the ectoderm and endoderm respectively are distinguished 
from one another, until a late stage of embryonic development, by the 
yolk-globules they contain ; for this purpose the use of the triple stain 
known as Biondi-Ehrlicli may be recommended. There is no homo- 
geneous tissue composed of cells wandering from the neural crest/ ectoderm 
* Anat. Anzeig., ix. (1893) pp. 51-6 (3 figs.). 
