THE EARLY DEVELOPMENT OF AMIA. 427 
a study of serial sections it may be determined that the central 
blastomeres are now separate from the underlying germ-yolk, 
but that the marginal blastomeres are unseparated from it; the 
nuclei still remain in the low region of the blastomeres. The 
dilated spaces separating the sides of the blastomeres might 
perhaps be regarded as artefacts. 
Fifth Cleavage (Pl. 30, fig. 6).—The stage of thirty-two 
cells is the first in which horizontal cleavages have been noted. 
These occur, however, only by variations in the divisions of 
the central blastomeres, and are by no means common. The 
typical conditions of this stage are shown in the above figure. 
From the study of living eggs the fifth cleavage was observed 
to take place in the following manner:—The five central 
blastomeres of the sixteen cells divide vertically in somewhat 
meridional planes, forming together a compact mass in the 
region of the animal pole, separated from the marginal 
blastomeres by a sharply cut trench ; a few minutes later the 
marginal blastomeres undergo vertical division in meridional 
planes. That this is the normal plan of cleavage has been 
verified by serial sections of the late sixteen-cell stages where 
the nuclear figures have been clearly followed. Thus it 
appears that there occurs a noteworthy difference from the 
normal mode of the fifth cleavage in Lepidosteus (Dean, op. 
cit.,p. 17). Variations, however, are numerous ; the horizontal 
cleavage of the central blastomeres has been already noted, 
and the tendency of the cleavage planes of the marginal 
blastomeres to pass obliquely seems to suggest an approach to 
the conditions of the gar-pike. This cleavage of Amia, there- 
fore, is not as nearly of the Teleostean plan as in the latter 
form. An interesting condition in this stage is the mode of 
origin of the newly formed marginal blastomeres ; these appear 
to be budded out directly from the germ-yolk (Pl. 31, fig. 25, 
gy.), their nuclei at first lying below the plane of the segmen- 
tation cavity ; the central blastomeres as before are separated 
from the germ-yolk by the segmentation cavity. 
Sixth Cleavage (Pl. 30, fig. 7).—The lineage of the cells 
of this stage could not be definitely followed; numberless 
