84 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



come under my observation where a nucleus projects into the cavity, as 

 Stauffacher ('03) figures it in his Tafel XIII. Fig. 19a. 



An interesting condition is found in Plate II. Fig. 16, in which the second 

 cleavage farrow 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, one between the cells A 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 in 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 con-elated with the size of the two quartets of 

 the fourth generation, Plate III. Figs. 20 and 21, and may be the occa- 

 sion of the frequent escape of the fluid 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 eggs in the living state, as well 

 as killed and hardened material studied both in toto and in sections, has 

 led me to the conclusion that this ephemeral and recurrent phase of the 

 cleavage cavity or blastoccel 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 

 by the development of normal embryos from vacuolated eggs. 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 

 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 in a quiescent 

 state, and the cells form a wall of such uniform thickness that it was 

 only after repeated trials that the vegetative pole of the egg could be 

 determined. The cavity is so large that the facets of contact are very 



