328 ROBERT CHAMBERS. 



echinoderm eggs which have generally been considered as naked. 

 The unfertilized Cumingia egg has an extremely tough mem- 

 brane, so tough that it is difficult to rupture it without com- 

 pletely destroying the egg contents. The vitelline membranes in 

 the frog and in the chick are undoubtedly analogous structures. 



3. THE EGG MEMBRANE AND THE FERTILIZATION MEMBRANE 



ARE IDENTICAL. 



Prior to fertilization no membrane enveloping the egg is vis- 

 ible. Upon fertilization a membrane lifts off which can easily 

 be cut away from the egg. Figs, n and 12 indicate the iden- 

 tity of a preexisting membrane with the fertilization membrane. 

 Fig. n-a shows an egg cut in two with an investing membrane 

 holding the pieces together. Upon fertilization the membrane 

 lifts off, enclosing the two pieces in a single cavity (Fig. n-&). 

 One only of the pieces happened to segment, and the fact that 

 the two pieces lie in one cavity is shown in Fig. ii-c, where the 

 blastomeres of the segmented portion have encroached on the 

 area around the nonsegmented piece. In Fig. 12 an egg was 

 cut into three pieces, the egg nucleus lying in one of the pieces. 

 Upon fertilization the membrane lifted, off the pieces, each of 

 which received sperm and developed into swimming larvse. Fig. 

 12-c shows the empty fertilization membrane after the three larvae 

 had escaped. In Fig. 13 is shown an egg which, on being cut in two, 

 was rolled about in an attempt to separate the pieces. The egg 

 membrane between the two pieces was twisted into a thread 

 joining the two. Upon fertilization each piece exhibited a com- 

 plete fertilization membrane, but the fact that the two investing 

 membranes are portions of one common membrane is shown by 

 the connecting thread. 



A conclusive test for the starfish and sand-dollar egg is the 

 removal of the egg membrane prior to insemination. Occa- 

 sionally, pricking the egg is sufficient to elevate the membrane. 

 No subsequent development takes place. It is possible, however, 

 to remove this membrane by tearing it and the egg then be made 

 to slip out. This is more easily done on eggs which have been 

 standing for some time in seawater. On catching at the sur- 



