QQ John H. Geroulu, 



those of Prostheceraeus vittatus, as described by von Klinckowsteöm 

 (1897) with the exception of the dagger-shaped elements of the latter. 

 This observer believes that the hooked chromosomes of ProstJ^- 

 ceraeus, which are similar to the slender rods with deflected terminal 

 knobs in Phascolosoma, have arisen from the elongated rings, which 

 are thought to open out, and thus to give rise to the hooked rods. 

 I agree with him that the rings give rise to the hooked rods, but 

 my observations differ from his in one important respect, viz. that in 

 Phascolosoma the rings break in the middle, making the division 

 transverse, whereas in Prostheceraeus a longitudinal splitting is said 

 to occur, which would necessitate a breaking apart and opening 

 outward at one end of the ring, tliough this has evidently not been 

 observed nor expressly stated to occur. Von Klinckowstrom's argu- 

 ment for longitudinal splitting, viz. that the daughter chromosomes 

 of the first maturation spindle are the the "mirror-images" of each 

 other is convincing if the elongated rings are unsymmetrical, but it 

 will not apply to Phascolosoma, in which symmetrical rings, breaking 

 apart transversely in the middle, also give rise to daughter chromo- 

 somes of identical form. 



Fourth period: Final Movements of the Pronuclei 

 and their Union (35—55 minutes after the approach of the 

 spermatozoon). 



At the end of the telophase of the second maturation division 

 (45 minutes), the eg^ nucleus consists of a cluster of ten chromatic 

 vesicles, which become fused together to form a single irregularly 

 oval body (Fig. 17, 18). The centrosome usually lies in a fold on 

 that surface of this pronucleus which is directed towards the centre 

 of the egg, and from it delicate radiations extend through the cyto- 

 plasm. This centrosome, surrounded by its gradually disappearing 

 radiating fibres, remains close beside the egg nucleus during the 

 final movement into conjugation of the two pronuclei. Probably 

 this centrosome and its aster never entirely disappear, though, while 

 the two pronuclei are approaching their place of meeting, it becomes 

 far less prominent; during this time it lies on one side of the egg 

 nucleus, about 90" from that point which is first to come into contact 

 with the sperm nucleus (Fig. 17 — 19). 



To return to the sperm nucleus : before the final slipping together 

 of the two pronuclei it lies in the centre of the egg, with its large 

 vesicular astrosphere, already described, on that side of it which 

 is directed towards the egg nucleus (Fig. 16—18). I have never 



