744 The American Naturalist. [August, 
This double plate of 16 cells is the blastula stage, the author affirms, 
and becomes a complete larva without rearrangement of cells, It fol- 
lows then that the normal animal pole of the egg is now found asa 
circular zone of cells about the equatorial periphery of the embryo while 
the vegetative pole of the normal egg is now represented by two 
isolated cells, one at each polar region of the entire mass; this is a 
disk that is becoming spheroidal by the conversion of the central parts 
which we have called its upper and lower surfaces into the peripheral 
zone or equator of the sphere. 
If the eggs are kept under pressure till 16 cells are formed these are 
found to be in a single layer. When now this plate of 16 cells is set 
free and a second division in accomplished there results a double plate 
of 32 cells having 16 in each layer. This seems almost identical with 
the above deseribed 32 cell stage and like it may form a complete 
normal pleuteus larva. As a matter of fact the two 32 cell embryos 
differ in the important item that cells having homologous positions in 
the two may have different relationships to the cells near them. Thus 
in the latter case cells at the periphery overlying one another, one in 
each layer, are brothers while like-placed cells in the former embryo 
are cousins. Another difference is that the 2 micromeres of the latter 
embryo do not divide into 4 equal cells but into 2 larger 2 smaller, as 
is the case in the normal embryo. 
From the above facts, which are illustrated by camera drawings, the 
author concludes that so fundamental a displacement of cells has been 
brought about that, first, some ectoderm arises from what would have 
been entoderm and second, that entoderm arises from what would have 
been ectoderm. The first conclusion is evident from Selenka’s observa- 
tion that the entodermal invagination is normally opposite the animal 
pule, or region of micromeres. For if these experiments are credited, 
the vegetative part of the egg is displaced into two separate sets 
of cells; later but one digestive tract arises, hence some of these two 
sets of vegetative cells must have taken part in the formation of 
ectodermal structures. : 
If we may assume that the micromeres are a determining influence, 
or that the entoderm must arise opposite them, even in these abnormal 
specimens, then the entoderm will be formed from a part of the 
abnormal egg that is purely of the animal pole or normally to form but 
oderm. 
Unfortunately the autbor has no observations upon these interesting 
stages of the abnormal larvze and we have no real knowledge of what 
. cells actually form the entoderm. 
