174 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
In several Mammals the most anterior cheek-tooth is not replaced. 
Does it belong to the first or to the second dentition ? In some cases it 
is preceded by a rudimentary milk-tooth, but in others, e. g. P. groen- 
landica , there is no distinct evidence of this. Yet from the nature of 
the tooth in question it seems to belong to the second set. But in other 
cases decision is difficult. It seems clear that each set or dentition 
should be regarded as a generation of teeth. The teeth which form the 
first set in higher Mammals form in more primitive types (in Marsupials, 
excepting Pm 3, and in Odontoceti) the only and the persistent set. 
Those of the first set retain ancestral characters which are lost in the 
second set. The rudiments of teeth belonging to the same set or gene- 
ration differentiate contemporaneously, or almost contemporaneously, 
from the enamel ridge. This is the real criterion. 
Fertilization in the Mouse.* — Dr. J. Sobotta has succeeded about a 
dozen times in observing the spermatozoon after it has penetrated into 
the ovum, and finds that the completion of the division which leads to 
the extrusion of the polar body does not take place until after the 
spermatozoon has entered. 
Development of Bladder and Urethra.| — Dr. P. Reichel has studied 
this in embryos of the pig. The inner cloaca formed by the ventral 
folding of the tail end of the embryo is divided by two folds from the 
lateral-posterior wall (Rathke’s folds) into a posterior narrow portion, 
the rectum, and an anterior wider portion which receives the Wolffian 
ducts and becomes the urinogenital sinus. The folds grow in and 
coalesce, the lower part of the cloaca is narrowed into the cloacal duct, 
the anterior main portion becomes the central part of the urethra and the 
bladder. Neither arise from the allantois, which opens into the upper 
part of the anterior bladder wall, in the pig by a canal which remains 
for a long time open, in man by a strand which has a narrower lumen 
and is soon closed — the urachus. 
The cloaca is closed by the anal membrane, a residue of the primitive 
groove ; this membrane grows into the cloacal septum, and the adjacent 
tissue grows into the genital tubercle. This is traversed from tip to base 
by the cloacal or urethral septum, which splits from behind forwards and 
forms the cloacal cleft or genital groove, thereby opening the cloaca. 
By fusion of the free, lip-like margins of the genital or urethral groove, 
the urethral canal is gradually formed. Clitoris and penis, clitoris- 
groove and the urethra of the penis are thoroughly homologous. But it 
is difficult to do more than indicate Reichel’s general position, so com- 
plex are the structural relations of these urinogenital parts. It is 
evident, at least, from his and other researches that there is a departure 
from the old view which derived the bladder from an expansion of the 
stalk of the allantois. 
Development of the Hypophysis.; — Herr E. Gaupp has studied this 
in Anguis fragilis and Lacerta agilis , and finds that the hypophysis has 
a triple rudiment (Anlage). The epithelium of the oral cavity gives 
rise to a large roundish “ median bud ” and two elongated “ lateral buds.” 
* Anat. Anzeig., ix. pp. 220-3 (1 fig.). 
f Verh. Phys. Med. Gescll. Wurzburg, xxvii. (1893) pp. 147-89 (3 pis.). 
j Arch. f. Mikr. Anat., xlii. (1893) pp. 569-80 (2 pis.). 
