466 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
Prof. Selenka * briefly replies to the foregoing. He uses the term 
inversion to mean that sinking-in of the embryonic ectoderm into the 
interior of the germinal vesicle during gastrulation which produces 
a temporary reversal of the position of the two layers, and considers 
this nse of the term as quite legitimate. The causes of the phenomenon 
may be various, and the described cases are not all due to the same 
cause. The method of formation of amnion, umbilical vesicle, and 
allantois, varies greatly, and is determined by the method and time of 
the fixation of the embryo ; the determination seems to him to be the 
result of mechanical causes. 
Influ enced ICold-on Frog’s Ova.j- — Prof. O. Schultze has made 
fresh experiments on the influence of low temperature on the develop- 
ment of the frog, and some of his 'results modify his previous con- 
clusions. 
A freshly fertilised ovum of Banc i fusca was injured by a tempera- 
ture of 0°, but no state of. quiescence due to cold ( Kalteruhe ) was 
brought about. 
Ova at the blastula stage were subjected for five weeks to a tempera- 
ture between zero and 1°, the development proceeded with extreme 
slowness to the gastrula stage, but after normal temperature was 
restored normal larvae resulted. 
Prof. Schultze proceeded to experiment with gastrula stages which 
he subjected to a temperature of about zero. The development pro- 
ceeded slowly (from April 18th to May 1st). Thereafter they were 
removed to the temperature of the room, and the result was four 
normal tadpoles, three malformations, and two dead eggs. Thus even 
in the gastrula stage development may go on at zero. It seems prob- 
able that in this case stoppage would mean death. In any case, the 
fact is that no thorough cessation of cell-division could be demon- 
strated. 
Behaviour -of Nucleolus during Development of Eggs.f — Herr 
Paul Obst has’ studied the changes exhibited by the nucleolus in 
developing eggs of some Molluscs and Arachnoids. He finds that in 
all the eggs examined, with the possible exception of those of Tegenaria 
domestica , two kinds of nucleolar substance appear during the process 
of development of the unripe eggs. The one (cyanophil) is the para- 
nuclein of O. Hertwig, and occurs at all stages of development. It is 
probably of more importance than the other (erythrophil), which appears 
later, and at very different periods in the various animals examined. 
The origin of the cyanophil is difficult to determine, but it seems to 
originate by the fusion of minute specks which appear in the nucleus. 
The cyanophil nucleoli seem to increase by taking up the erythrophil ; 
this is seen very distinctly in Epeira diademata. The author has no 
suggestion to offer as to the use of the two kinds of nucleolar sub- 
stance. The fate of the nucleolus during maturation was studied in 
eggs of Limax maximus , but the eggs were not double-stained. It was 
found that during the formation of the first spindle the nucleolus 
became vacuolated. Later the vacuoles coalesced to form one very 
large vacuole, the process being apparently a stage in the disappearance 
* Op. cit., xix. (1899) pp. 175-6, f Anat. Anzeig., xvi. (1899) pp. 144-52. 
X Zeitschr. f. wiss. Zool., lxvi. (1899) pp. 161-213 (2 pis. and 5 tigs.). 
