No. 2.] THE EGG OF BUFO LENTIGINOSUS. 329 



Fig. 44), from which a small pronucleus develops in about fifteen 

 minutes after the ^gg is fertilized. 



PI. XXXI, Fig. 48, shows a very young male pronucleus. It 

 is round in outline, measuring 0.008 mm. in diameter, and lies 

 in a small accumulation of cytoplasm directly behind the astro- 

 sphere which is preparing to divide. The male pronucleus has 

 at first an irregular reticulum surrounded by a colorless nuclear 

 sap. In its later growth it goes through the same processes 

 as described for the female pronucleus, so that when the two 

 pronuclei come together it is no longer possible to distinguish 

 between them. 



3. Growth and Division of the Astrosphere. 



By the time the entire spermatozoon has entered the Qgg, 

 the astrosphere has become an oblong structure composed of 

 a central uniformly granular portion, the centrosphere, from 

 which a constantly increasing system of rays extends out in all 

 directions (Fig. 44). Pigment granules which previously formed 

 a thick layer around the astrosphere now mark the course of 

 the rays, the yolk spherules occupying an inter-radial position. 

 At this stage the astrosphere greatly resembles the oblong 

 "attraction sphere " found by Zimmermann ('93) in the pigment 

 cells of Sargus annularis. 



The next changes in the astrosphere are shown in PI. XXX, 

 Fig. 47. The greater mass of the substance of the centro- 

 sphere has collected in two knob-like ends which are connected 

 by a narrow bridge of the same substance, the whole structure 

 appearing dumb-bell shaped. It contains numerous fine threads 

 which extend, for the most part, parallel to the longitudinal 

 axis of the centrosphere. The male pronucleus has formed 

 by this time and lies just back of the connection between the 

 two centers into which the astrosphere is dividing. The radial 

 system has become much more pronounced, the rays being 

 longer and more numerous and ending apparently in the 

 protoplasmic reticulum of the tg%. 



As the centers separate, they increase in size and the fine 

 threads of the centrosphere begin to arrange themselves in an 



