1893.] Embryology. 57 
EMBRYOLOGY. 
Gastrulation of Aurelia.*—Frank Smith has entered into the 
controversy between Claus and Goette concerning the origin of the 
entoblast of Aurelia. The results obtained from his first sections led 
him to think that the conclusions reached by Goette for Aurelia aurita 
would be confirmed in the case of A. flavidula. Better staining, 
thinner sections and more accurate orientation have, however, made it 
certain that the gastrulation in this species is much more in accord 
with the description given by Claus and that the process really is one 
of invagination. ‘The result of cleavage is a one-layered blastosphere 
as in A. aurita. The cells of the blastosphere are usually somewhat 
shorter at one pole than elsewhere, and it is from this region that the 
entoblast is formed. It develops as a single continuous layer of cells 
surrounding a small cavity, the coelenteron. From the beginning 
there is a narrow blastopore. Only a small portion of the wall of the 
blastosphere is concerned in the invagination, and to that extent it is 
not typical. The celenteron is, however, at all stages, an open sac- 
like cavity, and therefore noticeably different from that of A. aurita as 
described by Claus. The colenteron enlarges until the cleavage cavity 
is entirely obliterated and the entoblast everywhere comes into contact 
with the ectoblast. The entoblast, at first thin, thickens after the 
completion of gastrulation. 
While the entoblast is formed by invagination, ingression of cells 
from the wall of the blastosphere into the cleavage cavity does occur, 
although only in a minority of cases. It may happen any time after 
the blastosphere contains about 100 cells, sometimes before invagina- 
tion. When this phenomenon takes place it is similar to that repre- 
sented by Goette (Figs. 1-5) for the earlier stages of the blastula in 
A. aurita, and consists of the migration into the cleavage cavity of 
one or two, rarely more than three, of the cells of the blastospheric 
wall. Soon after invagination the nucleus of the cell disappears and 
the cell breaks down, or, less frequently, persists until gastrulation is 
complete. In the latter case it becomes forced through the entoblastic 
wall into the cavity of the ccelenteron. The cause or purpose of this 
immigration does not appear. 
ea department is edited by Dr. E. A. Andrews, Johns Hopkins Uni- 
versity. 
2Bull. Mus. Comp. Zool., Harvard, xxii. 
