No. 2.] THE EGG OF BUFO LENTIGINOSUS. 319 



separates sister-halves formed by a lofigitudinal splitting of 

 the chromatin rings, and there is an '* eqiial division'' in Weis- 

 matms ('92) sense. 



The second polar body (PI. XXX, Fig. 39) is extruded about 

 ten minutes after the spermatozoon has penetrated into the 

 egg. It is somewhat smaller than the first polar body, and 

 consists of a rounded mass of naked protoplasm enclosing the 

 twelve chromosomes and a few pigment granules, but no yolk 

 spherules. The chromosomes retain their individuality much 

 longer in the second polar body than they do in the first, but 

 eventually they fuse into a central mass which, as far as I have 

 been able to determine, never makes any preparations for a 

 further division. 



5. The Female Pronucleus. 



After the extrusion of the second polar body, the egg con- 

 tains but twelve chromosomes, one-half the number normal for 

 the somatic cells of the species. These lie in a small accumu- 

 lation of granular substance arising presumably from the break- 

 ing up of the spindle fibers which had already become granular 

 and decidedly irregular before the second polar body was 

 cut off. The chromosomes soon fuse into a rounded mass 

 (PI. XXX, Fig. 40), and from them the pronucleus is formed as 

 a small round body staining so faintly that its structure cannot 

 be determined. At a later stage, when its migration from the 

 periphery has begun (PI. XXXI, Fig. 49), the pronucleus has 

 increased greatly in size, and stains much more intensely. It 

 is either round or slightly oval, with a perfectly smooth mem- 

 brane which never shows any irregularities in well-preserved 

 specimens. The interior of the pronucleus is filled with a 

 clear, colorless nuclear sap, and a reticulum composed of irregu- 

 lar pieces of chromatin bound together by delicate linin fibers. 

 The reticulum becomes more regular as the pronucleus in- 

 creases in size (PI. XXXI, Fig. 50), and is then apparently 

 composed entirely of linin, the chromatin being collected in a 

 number of rounded masses which, for the most part, lie close 

 to the nuclear membrane or at the points of intersection of the 

 linin fibers. It is impossible to say whether all the deeply 



