40 THE EARLY DEVELOPMENT OF MAMMALS. 



Concerning the fate of the middle piece of the spermatozoon and its share 

 in the fertilization in the ovum of mammals, we possess no satisfactory informa- 

 tion. It has been shown, however, in other animals that this middle piece 

 produces a centrosome, and the only centrosome which appears in the fertilized 

 ovum. The theory has been advanced that the ovum, after its maturation, has 

 no centrosome, that a centrosome is always brought into the ovum by the sper- 

 matozoon in the manner just indicated. If we regard the centrosome as a perma- 

 nent cell element, then we must further interpret the addition of the male centro- 

 some as one of the most important phenomena of fertilization. Whether this 

 hypothesis is correct or not, we are unable at present to decide. 



FIG. 4. OVUM OF A WORM (SAGITTA) WITH Two FIG.' 5. OVUM OF A RABBIT, SEVENTEEN HOURS 

 PRO-NUCLEI. AROUND EACH PRO-NUCLEUS is AFTER COITUS, WITH THE PRO-NUCLEI ABOUT 



SHOWN THE ASTER. (After O. JETertwig.) TO CONJUGATE. 



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



Astral figures play a conspicuous part in the phenomenon of fertilization in 

 many animals. Astral figures are produced in the protoplasm of the ovum by its 

 assuming a special radiating structure. Astral figures may appear around both 

 the male and female pro-nuclei (Fig. 4). In other cases the astral figure arises 

 only in association with the head of the spermatozoon or male pro-nucleus. In 

 mammals, so far as known, no astral figures are developed about either of the 

 pro-nuclei. There is a clear space in the protoplasm around each nucleus, and 

 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 



