10 J. T. CUNNINGHAM. 
arise in the way that has been described above, it is possible 
to trace the steps by which the modifications of the segmenta- 
tion process exhibited by different vertebrate types have been 
reached. There has never been any difficulty in understanding 
how the unequal segmentation of the Amphibian ovum has 
been derived from the primitive regular segmentation; the 
presence of a certain amount of food yolk, which at a very 
early stage of segmentation is confined to the lower cells of 
the blastosphere, has caused these cells to be many times 
larger than the upper cells. The same is the case in the 
ovum of Petromyzon. In Lepidosteus and Acipenser (vide 
Balfour, ‘Comp. Embr.,’ vol. ii) we have the next modification. 
Some of the cells into which the yolk-containing portion of 
the blastosphere is divided are in these forms distinctly defined, 
but much larger than the corresponding cells in the Amphi- 
bian ; and others of the yolk-cells are incompletely separated 
from one another. In Teleostei, Elasmobranchii, and Sauro- 
psida the separation of the yolk-cells from one another has 
ceased altogether, and they form a syncytium. But in all 
cases the process of segmentation is essentially the same, and 
the first step in the process is not the separation of a yolk- 
containing cell from a purely protoplasmic cell, but the division 
of the ovum into two similar cells, each containing a cap of 
protoplasm and a large quantity of food-material. 
At the end of what may be called the period of simple 
segmentation the blastoderm in the ovum of the cod or haddock, 
has the form of a doubly convex lens resting in a shallow con- 
cavity of the yolk (figs. 3, 13). A thickened ring of periblast 
at the edge of the blastoderm is usually, as shown in fig. 13, 
conspicuous at this stage in optical sections, and it was this 
appearance of the periblast which first attracted attention to that 
layer. 
The Process of Invagination. 
The bi-convex blastoderm just described does not long retain its 
shape; it begins to extend laterally and to thin out at the 
centre. At the same time the central part raises itself up from 
the yolk, thus forming a closed cavity, the segmentation cavity. 
