ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 545 
enteron in front of the tip of the notochord ; it lies between trigeminal 
and facial rudiments and represents the beginning and posterior boun- 
dary of the cerebral curvature, not the infundibulum. 
While in Acanthias the premandibular and most anterior head- 
cavities alone arise among the emigrant cells which wander from the 
wall of the archenteron in front of the wings of axial mesendoderm, in 
NectUrus the mandibular mesendoderm also arises from cells which leave 
the wall of the archenteron in front of the axial mesendoderm. 
In the origin of the hyomandibular cleft, the mesendoderm is not 
broken through, as in the succeeding clefts ; the mandibular mesendo- 
derm grows down and bounds the hyomandibular cleft on which ecto- 
derm and endoderm are from the first in contact. The primitive rudi- 
ment of the trigeminal is chiefly formed from cells of the ectoderm 
lying above the brain ; from the manner in which the neural folds close 
in that region it is plain that some of the external ectodermic cells also 
take part in forming the rudiment of the trigeminal. 
Miss Platt describes in detail the origin and history of three longitu- 
dinal ectodermic ridges, which become connected by intersegmental 
transverse ridges. Behind the hyomandibular cleft there is a regular 
alternation of neural and mesendodermic segments. Regular interseg- 
mental enlargements of the gut suggest a long series of gill-clefts. 
The permanent connection of ganglia with brain arises at first from 
a centrifugal outgrowth of brain fibres. From the same ectoderm as 
that which gives origin to the dorsolateral mesectoderm of the trigeminal 
rudiment, the supraorbital line and the ophthalmicus superficial^ 
facialis arise. Therefore this branch of the facial cannot be brought 
into serial homology with any branch of the trigeminal. The cells 
from the neural ridge and from the dorsolateral and epibranchial thick- 
enings of the ectoderm form not nerves only, but share in the formation 
of the mesectodermic “ connective tissue ” and ganglia. 
Degeneration of Frog Ova.* — Sig. P. Mingazzini has studied the 
ovarian degeneration brought about by preventing female frogs from 
liberating their ova, in other words by removing the males. There is a 
rapid diminution in the size of the ovaries, and the size and number of 
the ova. Three stages of ova are described : (a) with incipient, ( b ) with 
well-advanced (c) with complete degeneration. The final appearance of 
the ovum is that of a darkly pigmented mass of cells with very areolar 
protoplasm and -with peripheral and radial connective fibres. The 
vitellus is chemically transformed into black pigments. Two kinds of 
elements help in the destruction, the epithelial cells of the follicle and 
the leucocytes. Both migrate inwards, feed on the degenerating vitellus, 
grow large and multiply by fragmentation. Mingazzini compares his 
results with similar observations on the degeneration of ova. 
Structure of the Nucleus in the Ovarian Ovum of Triton.f — Prof. 
G. Born gives a full account of his observations on the ovarian ova of 
Triton tseniatus. The nucleus of the primitive ovum exhibits the typical 
structure of a resting nucleus ; as the primitive ovum becomes definitely 
an ovum, the nuclear framework becomes a coil ; with further growth 
* Att.iR. Accad. Lincei (Rend.), ccxci. (1894) pp. 459-67. 
t Arch. f. Mikr. Anat., xliii. (1894) pp. 1-79 (4 pis.). 
