M. Ussow’s Zoologico-Embryological Investigations. 107 
opposite the narrow segments just mentioned, two very large 
segmentation-spheres are now separated by constriction 
(during the fourth hour) ; and these place themselves directly 
opposite to the two primitive spheres. In this way, in about 
four hours from the commencement of segmentation, there are 
produced eight segments and four spheres of segmentation. 
From these four, and ten subsequently produced spheres of 
segmentation, originates, by means of further spontaneous 
division (longitudinal division), the central part of the ger- 
minal disk. 
In the subsequent stages we observe the following :—1, a 
rapid multiplication of the central segmentation-spheres, a, 
by spontaneous longitudinal division, and, 6, by the rather 
rapidly advancing constriction of the apices of the segments ; 
and, 2,a multiplication of the segments by their slower longi- 
tudinal division. In this way, about the seventh hour of the 
process of segmentation, there are produced 10-12 radiating 
segments, whilst there are still only four central segmentation- 
spheres ; in the eleventh hour there are eighteen segments and 
at the same time fourteen segmentation-spheres (eight by divi- 
sion of the four above mentioned, and six newly constricted 
apices of the two longitudinal and four lateral segments). In 
the next (twelfth) hour a sphere is separated by constriction, 
by means of the so-called meridional segmentation, from each 
segment; all these spheres collect around those previously 
formed, and consequently at this stage the number of seg- 
ments amounts to eighteen, and that of the segmentation- 
spheres to thirty-two. Inthe next stage of segmentation the 
number of segments increases to thirty-two, which surround 
the germinal disk. But the latter now consists of 108-110 
cells, larger towards the periphery, smaller in the centre, 
which have multiplied in this manner by increased division. 
The number of nuclei of the segmentation-spheres and seg- 
ments likewise increases, a nucleus being contained in every 
sphere and in every segment. Both kinds of cells show no 
trace of a membrane; their finely granular protoplasm be- 
comes constantly darker, and is transformed from a trans- 
parent to a translucent substance. 
During the whole course of the process of segmentation the 
outwardly directed surfaces of all de segments, and especially 
of all the segmentation-spheres, are much raised, the highest 
being placed in the centre of the formative vitellus. At the 
close of the whole process, in the last stages, their convexities 
are far less observable; and finally the tubercular surface of 
the formative vitellus becomes quite smooth. As the final 
result of all these divisions, the one-layered germinal disk 
