HABITS, ETC., OF CERBBEATULUS LACTEUS. 139 



Hence the polar bodies on the exterior, where they are 

 plainly visible, repeat for us the same activities, or very 

 similar ones, that occur invisibly within the egg. We find 

 both bodies elongated at right angles to the first plane of 

 cleavage, while their ends, enlarged over each blastomere, 

 become centres of spinning activity. 



Externally the surface of each blastomere also manifests 

 spinning activity by rising into a papilla which becomes con- 

 nected with the enlarged end of the first body on its own 

 side. 



These visible filose activities of the egg are much more 

 prominent while the egg nucleus remains near the superior 

 pole, and diminish sensibly as it descends to meet the sperm 

 nucleus. 



Such a result would naturally follow the intervention of 

 inert yolk material between the active nucleus and the 

 surface of the egg. The increased activities of some eggs 

 immediately after the extrusion of the second polar body (2, 

 p. 232, and fig. 37), whereby the whole surface of the egg at 

 the superior pole is raised into ridges and papillae covered 

 with filaments, admit of ready explanation if we assume that 

 in such eggs for some reason the nucleus does not descend at 

 once, but tarries awhile close to the surface. 



Interoally the female pronucleus, after the extrusion of the 

 first polar body, rotates through 90'^, as already stated, so 

 that its ends are just beneath the points where the papillae 

 appear (fig. 56). 



This has been noted in Cerebratulus marginatus also 

 (16, figs. 20 and 22), where the amphiaster is plainly parallel 

 to the elongated polar body. Furthermore there is good 

 evidence for believing that the female pronucleus again 

 rotates after the extrusion of the second body, and becomes 

 parallel with the latter (fig. 57). But with the amount of 

 material at my command for this particular phase of deve- 

 lopment I have been unable to decide beyond a doubt. 

 After the egg nucleus has united with the sperm nucleus the 

 resultant segmentation nucleus is also elongated at right 



