MUSEUM OF COMPARATIVE ZOOLOGY. 493 



minute there is formed around the "head" the well-known stellate 

 figure. In the course of a few minutes the "head" advances to the 

 centre of the egg and remains there till the arrival of the egg nucleus. 

 Meanwhile the rays increase in number and in length, and at the same 

 time a "clear area," formed by the accumulation of protoplasm free 

 from granules, makes its appearance at the centre of the system around 

 the " head " of the spermatozoon. The " neck " of the latter gradually 

 swells until it attains one third the diameter of the egg nucleus. The 

 highly refractive tip of the spermatozoon is meantime thrown off and 

 borne away by the ever-active yolk protoplasm ; like the tail, it is ap- 

 parently resorbed. 



It always appeared as though a gentle amoeboid motion of the egg nu- 

 cleus begah only at the moment when the rays surrounding the sperm 

 nucleus had extended to it, as though it were thereby induced to begin 

 its migration to the centre of the egg, along the course marked out by 

 these protoplasmic rays. But in any event an automatic amoeboid mo- 

 tion of the egg nucleus must be maintained*. If it is granted that 

 definite courses for the streaming protoplasm are present in the yolk, (of 

 which, however, nothing is known with certainty,) it is not evident how 

 the nucleus could be urged into the centre of the yolk by such currents 

 of protoplasm, since the masses of the latter moving in centrifugal and 

 in centripetal direction must be equal, and since the egg nucleus would 

 therefore receive the same impetus in opposite directions. 



A direct union of the two nuclei follows, and is accompanied by very 

 active changes of form on the part of the egg nucleus, which sends out 

 thick pseudopodia-like projections enveloping the " Spermakern," and 

 then suddenly fuses with it. 



As soon as the point of the head of the spermatozoon has penetrated the 

 plasma mantle of the yolk there is raised up from the latter (within two 

 minutes) a fine membrane, which pushes before it the zona and absorbs 

 by diffusion the now fluid substance of the latter. In five minutes the 

 membrane is far removed from the yolk, and the zone is no longer 

 visible. 



The spermatozoa may penetrate the yolk at any other place than the 

 "Dotterhugel" without influencing the subsequent development, which 

 also continues to go on for a time in a quite normal manner, when 

 two, three, or even four spermatozoa at one time, or in quick succession, 

 penetrate the yolk at the "Dotterhiigel," or at different places on its 

 surface. In this case each "head" acquu-es independently its radial 

 figure. The author does not, in view of the observations of Fol and 



