52 



THE EARLY DEVELOPMENT OF MAMMALS. 



such a clear space has often been noted also when the astral figure is present. It 

 may possibly be interpreted as a rudimentary aster or center of astral formation. 

 The two pro-nuclei usually lie at some distance from one another. As soon 

 as they are formed, or perhaps when they are fully differentiated, they tend to 

 move toward one another and toward the center of the ovum. Concerning the 

 path of the male pro-nucleus we possess interesting information from the study 

 of the ova of the frog and axolotl. In these ova the spermatozoon leaves a trail 

 of pigment, which consists of two limbs, one passing through the cortical layer of 

 the ovum nearly perpendicular to the surface, and the other forming an angle 

 with the first and leading directly to the female pro-nucleus. The female pro- 

 nucleus tends always to move toward a central position. The force which draws 



FIG. 3. OVUM OF A WORM (SAGITTA) WITH Two 

 PRO-NUCLEI. AROUND EACH PRO-NUCLEUS is 

 SHOWN THE ASTER. (After O. Hertwig.} 



FIG. 4. OVUM OF A RABBIT, SKVENTEEN HOURS 

 AFTER COITUS, WITH THE PRO-NUCLEI ABOUT 

 TO CONJUGATE. 



p.g, Polar globules. (After Rein.} 



the pro-nuclei together is unknown. The hypothesis that this force is chemo- 

 tropic has met with favor. 



The fusion of the pro-nuclei. In the rabbit, as probably in all mammals, 

 both pro-nuclei lie at first eccentrically, but both move toward each other and 

 toward the center, meeting when the central position is attained. As they near 

 one another, both pro-nuclei perform active amoeboid movements. After they 

 meet they still continue their amoeboid movements, and travel together to the 

 center of the ovum (Fig. 4). One of the pro-nuclei assumes a crescent shape and 

 embraces the other. In the mouse the history is similar. After the two pro- 

 nuclei in this animal have met, they remain side by side, they are without mem- 

 branes, the chromatin threads become distinct and draw closer together. Between 

 them appears first a small spot or centrosome with a few radiating lines between 

 it (Fig. 5). From the centrosome arises a spindle of achromatic threads (Fig. 6). 



