73 
eludes that the embryology of the oyster and that of other 
Metazoa proves that the starting point in their development 
is not the gastrula stage, but a “ planula ” stage, in which the 
embryo consists of two layers of cells without a central cavi- 
ty ; that this planula stage may be so modified as to give rise 
to a “blastula” stage, in which a double layer of cells sur- 
rounds a central cavity without an external opening ; that 
wdiile the blastula is to be regarded as a modification of the 
planula, the planula stage may be omitted, and the embryo 
may at once assume the blastula form ; that the formation of 
the stomach-cavity is a later and secondary occurrence in the 
history of development, that it takes place at different stages 
in different animals, and has no place in a conception of the 
general plan of development ; that either the planula or the 
blastula may complete its development by passing through a 
“gastrula ” stage, which, however, is not the primitive condi- 
tion of the embryo, but a secondary modification of later for- 
mation, which may or may not present itself ; that the “ gas- 
trula ” stage of development is not the common starting point 
for all Metazoa ; and that the hypothetical “ Gastraea ” can- 
not be regarded as the ancestral form from which the higher 
groups have been derived, pp. 159-173. 
A comparison of his figures of the oyster embryo with my 
own shows that his Figure 1 represents a stage between my 
Figures 36 and 37 ; his Figure 2 one at the same stage as my 
Figure 38, and his Figure 3 one at the stage as my Figure 45. 
He is correct in the statement that the embryo shown in 
his first figure consists of a central mass of endoderm which 
is entirely surrounded by the ectoderm, and in which no cavi- 
ty can be seen ; and that the mouth and anus are formed and 
the stomach-cavity becomes visible at a late stage, after the 
shell and velum have appeared. 
My own observations, and those of Habl on TJnio, show, 
however, that the so-called “planula” of the oyster is pre- 
ceded by a true invaginate gastrula stage, and that, so far as 
the development hypothesis above quoted rests upon the em- 
bryology of the oyster, it is directly opposed to the facts. 
