144 BERTRAM G. SMITH 



whereby the middle-piece is brought into position to precede 

 during the further process of migration. Fick ('93) has described 

 this process in Axolotl, and Dean ('06) has noted it in Chimaera. 

 King ('01) found no indication of a rotation of the sperm head 

 in Bufo; possibly this condition is correlated with the fact that in 

 Bufo the centrosome appears to be located, not in the middle- 

 piece, but in the head of the spermatozoon. In Cryptobranchus 

 rotation of the sperm head apparently takes place at a rather late 

 stage in the process of transformation into the sperm-nucleus. In 

 the stages shown in figures 43 and 44, the greatly shortened sperm 

 head is usually placed with its long axis oblique or at right angles 

 to its former path, so that one end points toward the egg nucleus. 

 But in these stages it has not been possible to trace any connec- 

 tion between the tail of the spermatozoon and its head, and since 

 the aster has already separated from the sperm head, in no case 

 can it be stated which end of the sperm head is the one pointed 

 toward the egg-nucleus. 



The spermatozoon ordinarily enters the blastodisc in a more or 

 less centripetal direction, and continues in this direction for a con- 

 siderable distance; sometimes its path inclines almost from the 

 beginning in an oblique direction toward the point of future union 

 with the egg-nucleus. In either case the axis of the spiral path 

 is ordinarily straight up to the time of the transformation of the 

 head of the spermatozoon into the sperm nucleus; the later course 

 of migration has not been followed. In an egg preserved an hour 

 and three-quarters after fertilization, a spermatozoon, which had 

 entered the blastodisc unusually near the animal pole, described 

 a path which proceeded in a centripetal direction only a very short 

 distance, then curved sharply in a direction parallel to the sur- 

 face, toward the second polar spindle which was in the late ana- 

 phase condition. The form of the spermatozoon remained unal- 

 tered, and rotation of the sperm head had not commenced. This 

 case is instructive in showing that the factors tending to bring the 

 germ-nuclei together are active at a very early stage of fertiliza- 

 tion: the egg-nucleus was not fully formed, and the spermatozoon 

 had not begun its process of transformation into the sperm- 

 nucleus. Moreover it is evident that in this case at least the 



