A. A. W. HUBRECHT. 
[or 
Fresh eggs have served for the observation of the cleavage- 
processes in the rabbit to van Beneden and in the bat to van 
Beneden and Julin. Most of the other authors have made 
use of preserved specimens and of sections. A certain 
number of the figures given by different observers are here 
reproduced (Figs. 1—36). 
1. The Mammalian Morula. 
The compact mulberry stage (different in its compactness 
from the hollow mulberry of the holoblastic egg of Am- 
phioxus alluded to above) contains about 36—72 cells. In 
the case of Tupaja and—judging from Bonnet’s figure—of 
the dog the central cell or cells show a different reaction 
against staining reagents than the peripheral (Figs. 1, 2, 3). 
We will have occasion to discuss this phenomenon later on. 
Very soon fluid begins to accumulate between some of the 
constituent cells of this mulberry stage in mammalian de- 
velopment, and the solid mulberry then becomes converted 
into a hollow sphere, against the wall of which an accumu- 
lation of cells is visible which was already noticed by Bischoff 
(42, 45) and other early authors. 
Twenty-five years ago, when van Beneden published his 
remarkable researches above alluded to on the early develop- 
ment of the rabbit, the interpretation of these early pheno- 
mena was far from being satisfactory or uniform. The so- 
called metagastrula stage of mammals, first described by 
van Beneden (’80), has since been abandoned by that author 
[though taken up again by Duval (’99, p.64)1._ We may, how- 
ever, say that of late years a very general consensus of opinion 
has come to be established. In all the orders above-mentioned 
an early stage of the blastocyst has been observed corre- 
sponding to the phase just described in which an accumula- 
tion of cleavage-cells adheres at one point against an outer 
epithelial layer.' 
‘ KE. van Beneden has ascribed the origin of the free space between the 
