INOm 2a AGG (OTE ANIMA. AUNT AG Sae Cis IZA V AGE. 335 
Passing over the singular construction of the second state- 
ment, which we confess to being utterly unable to comprehend, 
we are led to make a remark on the “segmentation cavity.” 
It seems evident that Dr. Dean is predisposed to interpret his 
sections of the Amia egg in correspondence with what he finds 
laid down by Dr. H. V. Wilson in regard to the egg of Ser- 
ranus. The discovery of a ‘segmentation cavity” in his 
Fig. 23 seems to have been thus inspired (compare this figure 
with Wilson’s Fig. 15). 
If the section shown in Dean’s Fig. 23 passes in the plane 
‘indicated in his Fig. 4, the cell- zis cut very near ats central 
margin, and a little obliquity, either in the plane of the section 
or in the plane of the bounding groove, might be sufficient to 
account for the appearance. In other words, the section may . 
represent only a chip from the central margin of cell a. 
We must differ with Dr. Dean in respect to what seems to 
be his notion of “the segmentation cavity.’’ The fissure, or 
fissures, thus designated by Dean, as a glance at his Figs. 27— 
30 will show, represent nothing more than ordinary inter- 
cellular spaces. Such spaces, as all the world knows, are not 
peculiar to any egg, and are not to be confounded with ‘“ ¢he 
segmentation cavity.” Dr. Dean says in one place (p. 424) 
“the segmentation cavity is practically wanting.” Neverthe- 
less he speaks later of its “definite origin,’ and calls special 
attention to it in his figures. In one case (p. 441) it is called a 
“flattened segmentation cavity.” 
Dr. Dean represents the segmentation cavity as beginning 
with the second cleavage but as taking “ definite origin”’ at the 
time of the third cleavage. In Dean’s Fig. 24, representing a 
section of an egg in the stage of the fourth cleavage, the “‘seg- 
mentation cavity’ is shown as intercellular spaces, which he 
admits “might perhaps be regarded as artifacts.’’ We would 
suggest that these. spaces have a different meaning. The 
elongated nucleus shown in one of these cells is very clear 
evidence that what we have called the horizontal cleavage is in 
progress. At the time of the cleavage a constriction takes 
place at about the level of these spaces, and it is to these con- 
strictions that they probably owe their origin. There is another 
