542 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
cells of the ovary. During their growth, which affects all parts of the 
cell, the chromatin elements become more distinctly arranged and ex- 
hibit a considerable increase in mass. After the primitive ovum has 
attained a certain size, the formation of the tunica adventitia and follicle 
begins. These are described, and Prof. Holl goes on to discuss the 
series of changes in nucleus, cell-substance, and follicle which constitute 
the process of maturation. The ovum increases in size ; the nucleus and 
nucleolus likewise ; at a certain limit the nucleolus begins to be 
emptied and is broken up ; the network character of the chromatin is 
lost, the knots of the net disappear, chromatin balls, which form the 
loops of the directive figure, appear, seemingly from the Schroen’s 
granules of the nucleolus. It is difficult to reach certainty in regard to 
the history of the network. The chief changes in the cell-substance are 
increase in size and deposition of deutoplasmatic elements. The 
changes in the tunica and follicle are also described. 
Segmentation in Medullary Folds and Embryonic Rim. * — Prof. 
W. A. Locy begins an interesting communication by sketching the pro- 
gress of knowledge in regard to the segmentation of the neural tube. 
He then describes the stages which he has observed in Squalus acanthias , 
where he finds the primitive metamerism in very much earlier stages 
than as yet recorded for other animals. The division into epiblastic 
segments is very distinct long before the closure of the neural groove 
(between Balfour’s stages C and D), and it extends not only the whole 
length of the embryo, but also some distance into the embryonic rim. 
It is clearly defined throughout the whole length before the mesoblast 
has, to any extent, become divided into somites. The fact that the 
primitive segments extend into the embryonic rim, and are subsequently 
drawn into the axial embryo, supports the doctrine of concrescence that 
the germ-ring represents, or originally represented the divided halves of 
the embryo, and that it is formed in part by their apposition. 
Succession and Genesis of Mammalian Teeth. | — Mr. M. F. Wood- 
ward summarizes our present knowledge of this subject, on which, as 
readers of this Journal will be aware, many workers have lately been 
engaged. The most remarkable result is the conclusion to be drawn 
from the work of Leche, Kiikenthal, R5se, and others, that there 
appear to be traces of four dentitions in Mammals, viz. : — 
1st or pre-milk dentition, minute calcified teeth never functional, 
present in Myrmecobius. 
2nd or milk dentition, generally functional, the permanent dentition 
of Marsupials and Cetacea. 
3rd or replacing dentition, functional in most Mammals. 
4th dentition (?), rudimentary in the seal (?), occasionally functional 
in Man (?). 
Milk Dentition of Rodents. J — Mr. M. F. Woodward records the 
occurrence of vestiges of an early set of incisor teeth in the common 
mouse. “ The squirrel, the rabbit, and the mouse present us with all the 
stages in the suppression of the vestigial milk incisors ; in the former 
* Anat. Anzeig., ix. (1894) pp. 393-415 (11 figs.). 
t Science Progress, i. (1894) pp. 438-53. 
X Anat. Anzeig., ix. (1894) pp. G 1 9-31 (3 figs.). 
