FERTILIZATION STAGES. 49 



is only demarked by a slight furrow. Under a dissecting lens a number of minute 

 depressions indicate the points of entrance of sperms. Seven of such points appear 

 in the present instance, and all of them are peripheral; four are close together. In 

 this case sections show that no sperms have entered the middle of the germ. 



A middle stage in fertilization (plate iv, fig. 19), also examined in the living egg, 

 showed 23 entrance pits. Of these half a dozen are of large diameter and several 

 are minute, a condition which, in comparison with the preceding stage, suggests 

 that the small pits are the early phases of the large ones, and we querj', accord- 

 ingly, whether in point of time the entrance of sperms in Chima^ra may not prove an 

 extended process (v. infra, heading /). In the present specimen it will be seen 

 that the sperms have entered not only the germinal substance but the bottom and 

 even the outer wall of the germinal fosse. 



Study of sections leads us to conclude: 



{a) That the tail of the spermatozoon does not enter the egg. In fig. 35 a 

 sperm is shown which has just entered the egg; the middle piece, mp, ends 

 abruptly, and there is no trace of the tail. The entrance pit is not yet sharply 

 formed. 



(/;) That the head of the spermatozoon rotates as it travels inward. Even at 

 the early period above figured, the filamentous character of the sperm head has 

 been lost ; it is now sjiheroidal, surrounded b}- a light-colored area of the germ. 

 Although hardly within the egg, its axis inclines 45° to the surface, and its middle 

 piece is parallel with the surface, a condition which by analogy with other forms 

 leads us to conclude that it has already begun a process of rotation. In a later 

 stage in the entrance of the sperm (fig. 38) the lighter-colored portion of the "head " 

 points toward the surface of the germ and thus indicates that the rotation has been 

 carried through an angle of 180°. 



if) A state of remarkable kinetic activity exists in these stages. In fig. 36 a 

 series of "astral rays" are seen diverging downward from the entrance pit of a 

 spermatozoon {cf. the observations of Miss Foote in Allolobophora). And from paths 

 traversed by a sperm "astral rays" arise, sometimes radiating regularly, but usually 

 branching irregularly and forming new groups of radiation. At such points of 

 reradiation darkly staining bodies occasionally appear which remind one of centro- 

 somes. In the present fig. 34 branching astral rays are seen. These, it is found, 

 have arisen around a sperm path. A similar series greatly enlarged is shown 

 in fig. 37, a series of considerable interest, since it shows many "centrosomes" 

 surrounded by bending and irregularly branching rays. The "centrosomes" some- 

 times appear at centers of reradiating rays in sperm asters (figs. 38, 39); at other 

 times they arise without any apparent relation to sperm asters or sperm paths, as 

 around an unusually large yolk granule (fig. 40, the group at the right). As shown 

 in the last figure, more than half a dozen centers of radiation appear around the 

 yolk granule. On the other hand, the two large "asters" shown at the left in the 

 present figure have no apparent relation with the former series, nor are they in the 



