FERTILIZATION STAGES. c, 



which is pictured a (sperm) merocyte occurring eccentrically () in the germinal 

 area of an egg twin to the preceding. In this the vacuolar area has been practically 

 lost, the aster increased in size, and the nucleus subdivided into a number, probably 

 five or six, of smaller merocytes. Division of this kind has been observed in many 

 instances; and on the other hand no case has been found in which a sperm nucleus 

 divides indirectly. This condition is noteworthy, since it emphasizes on still 

 another line the specialization of the Chimseroid. For in the shark the sperm 

 nuclei may undergo indirect division throughout practically the entire process of 

 cleavage ; and when early direct divisions do appear, e. g. , in the third cleavage 

 (Ruckert in Torpedo), they still show traces of their mitotic ancestry. Indeed, the 

 nearest condition to the presegmentation division of the sperm head in Chimaera 

 (fig- 39) occurs in shark only in the period of later segmentation (cf. Ruckert, op. 

 cit., pi. in, fig. 1 8). In other words, the morphological (or the physiological) 

 result which in the shark is effected only at the end of a series of graduated stages 

 is accomplished by Chimsera at a single stroke a condition worthy of comment, 

 since it affords a palpable case of "precocious segregation." 



(_/) The sperms enter the germ not simultaneously, but during a relatively 

 extended period. The pits formed by the sperms when entering the germ, as already 

 noted, are different in size, and we accordingly infer that, as the sperms themselves 

 do not differ materially in size, nor in all probability in individual activity, the dif- 

 ference in the pits is due to their having been formed at successive periods. This 

 suggestion is borne out by examination of sections. Thus, in fig. 36 an entrance 

 pit is shown, pointing down in the direction of, but not actually connected with, a 

 sperm head lying deep in the germ. And here the pit or funnel has a wide mouth. 

 On the other hand, in fig. 38, a funnel is pictured whose apex is still connected 

 with a sperm head, and its mouth is narrow. The sperm head, in this case, lies 

 in a shallower layer of the germ, and from its structure, also, is clearly a younger 

 stage in development. It follows, therefore, that the former sperm entered 

 the germ at an earlier period than the latter, and that the process of semina- 

 tion is a relatively extended one relatively, since in sharks all sperms appear 

 to enter simultaneously. The suggestion may, on the other hand, be made that 

 the difference in the behavior of the sperms in the germ might be due rather to 

 their location than to their time of entrance; or, in other words, that the rapidity 

 of their development might be influenced by their proximity to the egg nucleus. 

 This suggestion, however, is not tenable in view of the condition of the fertilization 

 stage (middle stage) shown in plate iv, fig. 19, for here small pits occur side by side 

 with large ones, both in the middle of the germinal area and on the sides. 



Finally, to contrast Chimera and shark in stages of fertilization: In Chi- 

 msra the entrance of the sperms is a protracted process; but as soon as the sperms 

 (other of course than the one which fertilizes the egg) enter the germ they divide 

 promptly by amitosis, with the very probable result of producing a greater number 



