84 BULLETIN: MUSEUM OF COMPARATIVE ZOÜLOGY. 
come under my observation where a nucleus projects into the cavity, as 
Stauffacher ('03) figures it in his Tafel XIII. Fig. 19 a. 
An interesting condition is found in Plate IT. Fig. 16, in which the second 
cleavage furrow is almost completed. The cavity appears to have been 
divided into two parts by the recent cleavage furrow, and now consists of 
two large lenticular spaces, ono. botween the cells 4 and D, the other 
between B and C, i. e. both spaces are in the first cleavage furrow. ‘The 
first appears to lie mainly in the cell A, but this is due to the fact that 
A lies slightly above and upon D. The cavity between B and C has 
several secondary contributory spaces lying superficially to it iu the 
furrow at the animal pole. 
The cavity of the eight-cell and later stages differs from that of the 
two-cell stage in that it is situated nearer the animal than the vegetative 
pole of the egg. This is correlated with the size of the two quartets of 
the fourth generation, Plate IIT. Figs. 20 and 21, and may be the occa- 
sion of the frequent escape of the flnid contents at the animal pole. 
It is not necessary to follow in detail the phenomena which attend 
the further history of the cleavage cavity, as it would be in the main a 
repetition of the description of that of the earlier stages. I shall merely 
call attention to certain features of the cavity which are of especial 
interest. 
An examination of a large number of eges in the living state, as well 
as killed and hardened material studied both 4» toto and in sections, has 
led me to the conclusion that this ephemeral and recurrent phase of the 
cleavage cavity or blastocel continues until a late stage, even to the 
period of gastrulation. That its appearance is not due to a pathological 
condition of the embryo is shown by the prevalence of the same phenom- 
enon in eggs collected in the natural environment of the slug, as well as 
gos, It may 
be that confinement conduces to the presence of the ephemeral cavity 
in its various forms, but I have no direct evidence that such is the 
by the development of normal embryos from vacuolated e 
case. 
Eggs presenting the maximum development of the cleavage cavity in 
the later stages are with great difficulty freed intact from their enve- 
lopes and require especial care in the subsequent treatment with reagents. 
On Plate V. (Figs. 33 and 34) is figured an egg of twenty-four cells 
with a well developed cleavage cavity. The nuclei are all ın a quiescent 
stato, and the cells form a wall of such uniform thiekness that it was 
only after repeated trials that the vegetative pole of the egg could bo 
determined. The cavity is so large that the facets of contact are vory 
