172 bulletin: museum of comparative zoology. 



Another egg (Fig. 15) shows essentially similar conditions, and 

 renders the above explanation more probable. In this ease a small 

 body is found on the surface of the egg, inside the envelope which 

 surrounds the egg. A dark mass within this body probably represents 

 chromatin, the entire body being a polar cell, which agrees in position 

 with the polar cell shown in Figure 13, though somewhat smaller. 

 The chromatin left in the egg after the formation of the first polar cell 

 has been reorganized in this case into several vesicles, each with a 

 definite membrane; a reticulum has been formed in each vesicle, with 

 the chromatin more or less scattered, but not so diffusely as in Figure 

 14. Radiations, similar to those found at the peripheral end of the 

 nuclear body in Figure 14, are present here also, though not so plainly 

 marked. At the inner border of the group of more deeply lying vesicles 

 are faint astral radiations. These radiations probably represent the 

 asters at the poles of the forming second maturation spindle. 



Since these are the only instances of such conditions which were 

 found, it is not possible to say whether such a stage in the formation 

 of the polar cells is typical. However, this lack of evidence is not a 

 fatal objection to the interpretation given, since stages showing matura- 

 tion spindles are almost as rare; indeed, only one egg showed a second 

 polar cell in process of formation. Moreover, it is significant that 

 conditions similar to these are quite common in Tubularia, though 

 the polar cell was not found in these cases. 



The earliest stages in the reorganization of the chromosomes into 

 an egg nucleus, after the formation of the second polar cell, I have not 

 succeeded in finding. Somewhat later the egg nucleus is represented 

 by several vesicles (Plate 3, Figs. 21, 22). Each of these contains chro- 

 matin in a network of linin, though often the chromatin is collected 

 most abundantly at the nodes of the net or close against the nuclear 

 membrane. Sometimes these vesicles soon fuse with one another, 

 but they may remain separate and distinct till about the time of their 

 union with the sperm nucleus. 



To sum up: polar cells appear to form just before, or at the time 

 of, liberation of the medusae. The concentration of chromatin in 

 preparation for the production of chromosomes is not coincident with 

 the disappearance of the nucleolus. The germinative vesicle becomes 

 ellipsoidal, asters with centrosomes appear, and the maturation 

 spindle forms, in part at least from the cytoplasm, before the mem- 

 brane of the germinative vesicle disappears. The first maturation 

 spindle is at first tangential to the surface of the egg, and contains the 

 reduced number of chromosomes, probably ten. A splitting occurs, 



