150 Gastrulation and Embryo Formation in Amia Calva 
which the lenses are present, but not shown. Behind these in turn are 
three gill clefts (v. ¢.) and between them the mesoblastic bases of the 
corresponding arches. Close to the medulla are the auditory vesicles 
(a. v.). Around the periphery the lnes of darker cells represent the 
hypoblastic walls of the gut (g.) The section is cut so thick that the 
mesoblast shows above, making it almost appear as if the gut were filled 
with these cells. 
SUMMARY AND GENERAL REMARKS. 
THE SEGMENTATION CAVITY. 
Before considering the stages which properly belong to the present 
paper, we are obliged to say a few words concerning the segmentation 
cavity. Whitman and Kycleshymer, 96, pointed out that there are to be 
found in the egg, even in the earliest cleavage stages, certain irregular 
cavities which sooner or later become continuous with the cleavage 
grooves and in many cases unite to form a common cavity. These cavi- 
ties appear in eggs collected in different years and in different seasons 
of the same year and fixed and imbedded in various ways. Since it is 
from these spaces that the segmentation cavity later takes its origin they 
have been subjected to renewed study. As segmentation progresses the 
cleavage grooves, in many cases at least, expand into broad spaces as they 
approach the center of the egg, in which locality they become continuous 
with the earlier spaces described above. Often these cavities unite and 
give rise to a more spacious one as figured by Whitman and Eycleshymer. 
That the cavities should be regarded as artifacts seems highly improb- 
able. In the first place no cellular fragments are to be found in the 
spaces which would indicate imperfections in cutting. Again these cavi- 
ties often shade off by imperceptible degrees into veritable intercellular 
spaces which no one would consider as artifacts. As the later stages of 
the blastula approach, the cavities no longer show as large irregular 
spaces, but become more or less obliterated by being filled with the 
rapidly proliferating cells of the blastodise and yolk. In view of these 
facts, we cannot agree with Sobotta, 97, that these cavities are artifacts. 
PERIBLAST. 
The periblast in ganoids was first discussed by Dean, 95, 96, who 
pointed out the homology of the upper layer of yolk cells in Acipenser, 
Lepidosteus and Amia with the periblast of teleosts. Sumner, oo, later 
gives two figures showing the periblast in Amia. In one of these (Fig. 
16) he represents a well defined, clear zone lying on the large yolk masses 
