288 W. G. VcmJVariie — Embryology of Eustylochus. 



As has already been described, the spermatozoon quickly penetrates 

 to a considerable distance from the surface of the egg and appar- 

 ently remains practically stationary until maturation is complete. 

 When the first polar spindle has taken its final position and we can 

 recognize the animal pole of the Q^g, we find that it more often lies 

 in the hemisphere away from the animal pole and has evidently 

 entered at some point on the sui'face of that hemisj^here. This state- 

 ment however applies to Eustylochus only ; in Planocera it is perhaps 

 more frequent to find the sperm-nucleus in the upper hemisphere. 

 In this species it usually lies closer to the center of the egg and con- 

 sequently often very near the polar spindle aster (Figs. 38 and 40). 

 This more central position is often noticeable even in uterine eggs 

 (Fig. 36). 



The movements which bring the pronuclei together seem to be 

 chiefly movements directed toward the center of the Qgg., both on 

 the part of the male and the female. Just before the formation of 

 the cleavage spindle we find them usually quite near together in the 

 central part of the Qgg, the female nearer the animal pole. 



The Polar Bodies. 



During the pronuclear stages the polar bodies usually lie side by 

 side, touching each other and the egg, showing that the point of 

 separation of the second is nearly but probably not quite coincident 

 with that of the first. There is generally considerable difference in 

 the ai-rangement of the chromatin in the two polar bodies. In the 

 first it remains in a compact group, which often appears to be com- 

 posed of beaded threads. In the second, however, the rounded 

 chromosomes are scattered irregularly in the cj^toplasm of the polar 

 body and are often widely separated. This difference is suggestive of, 

 and doubtless largely due to, the form and arrangement of the chromo- 

 somes in the later stages of the first and second polar spindles 

 respectively. The first polar body rarely if ever divides. 



The Sperm-centrosomes and their Asters. 



These are shown in a number of my specimens of Planocera 

 nehulosa, though the examples illustrated (Figs. 38, 39 and 40) are 

 cases where they are visible with unusual clearness, I attribute my 

 failure to find them in Eustylochus to the same causes which make 

 them indistinguishable in many jDreparations of Planocera iiebulosa. 

 The radiations are veiy few and short as well as very weak and 



