FORMATION OF GERM-LAYERS IN ORANGON VULGARIS. 3847 
128 cells are present, the cell outlines still extending nearly to 
the centre of the egg. The median section drawn in fig. 12 
shows that the central yolk-mass is at this stage composed of 
elements similar to those of the rest of the yolk. After this 
period the regularity appears to be lost, for in the next stage 
I have only been able to count 175 nuclei. 
During the later stages of segmentation the nuclei, with 
their surrounding protoplasm, approach more and more closely 
to the surface of the egg, while the segmentation furrows 
become less and less distinct. In the stage with 175 nuclei 
the furrows can only be traced through about half the distance 
from the periphery of the egg to the centre (see fig. 18); and 
at the same time the central yolk exhibits a marked tendency 
to run together into large, irregular masses under the influence 
of reagents. 
The later stages of the segmentation have not been followed 
in detail. At the close of this period the egg is surrounded by 
a layer of cells which are, except in one small region, uniform 
in character ; while the yolk is devoid of nuclei. At one point, 
which marks the posterior end of the future embryo, is a 
shallow depression (fig. 5, Blp.) lined by cells which are richer 
in protoplasm than their neighbours, and which are about to 
divide and to pass again into the yolk. 
Professor Kingsley’s account of the segmentation differs in 
many important points from that which is here put forward. 
According to this observer, the segmentation furrows are 
throughout confined to the surface of the yolk, so that the 
greater part of the substance of the egg remains permanently 
undivided. At the stage with “about sixteen cells,’ he 
describes and figures (first paper, p. 106, and fig. 4) a central 
nucleated mass of protoplasm in the position of the mass de- 
scribed above as unsegmented yolk. He says: 
“ As will be seen from fig. 4, most of the protoplasm has 
reached the surface of the egg, but there still remains some 
near the centre of the yolk. Whether this is the same as the 
protoplasm described by several authors ... . I cannot say ; 
but I am certain not only that it is derived from the first seg- 
