48 Papers from the Marine Biological Laboratory at Tortugas. 



ence of the postsynaptic chromosomes (sometimes still partially arranged in 

 a thread). Thus the chromosomes have remained throughout the growth 

 period in small individual bulk (always retaining their morphological iden- 

 tity) and in a compact mass. Their proximity to the nucleolus is to be 

 explained in connection with the maturation phenomena. 



Occasionally I have seen very close to the nuclear wall a small dumb-bell- 

 shaped body (figs, 40, a, 49) (sometimes the body consists of three or even 

 four globes) somewhat larger than the ordinary bivalent chromosomes. 

 Mathews (1895) describes very definitely the origin of the centrosome from 

 within the nuclear membrane. I have tried to identify this problematical 

 body with the centrosome of Mathews. This I am unable to do for several 

 reasons. Bryce (1903) also describes very similar bodies in Echinus escu- 

 lentus, but savs that he has not been able to convince himself " that thev are 



v> f 



more than accidents of staining and fixing" (p. 491). Hartmann (1902) 

 makes no definite mention of such structure in Asterias glacialis, probably 

 including this as well as the above-mentioned masses of granules under the 

 " clumps of chromatin " and " accessory nucleoli." However, eggs in which 

 such distribution of chromatin occurs Hartmann classifies as " abnormal," 

 stating further that in normal eggs all " genuine chromatin and plastin were 

 combined in a single nucleolus." Nor does Guenther (1903) mention this 

 body in Psammechinus microtuberculatus or Holothuria tubulosa. I tried 

 also to identify it with portion of the chromosome mass, but unsatisfactorily. 

 Though I can make no positive statement concerning it, inclining to the 

 opinion that it represents several or perhaps a single large bivalent chro- 

 mosome, since it is not invariably present with certainty, I am convinced 

 that it can not be the centrosome. For reasons soon to be given, I hold to 

 an extra-nuclear origin of the centrosome. Furthermore, this problematical 

 body is always at least double, while the centrosome arises as a single 

 structure. Again, its size is several times larger than the largest centro- 

 some I have seen during the maturation process. I have not been able to 

 follow satisfactorily the fate of these bodies amid the general mingling 

 and concentration of chromosomes and nuclear fragments and their exit 

 from the nucleus at one point during the initial stages of maturation. 



MATURATION. 



NUCLEAR AND CYTOPLASMIC ALTERATIONS. 



Study of sectioned material confirms in every respect my observations on 

 the living eggs. In batches of eggs fixed from 5 to 10 minutes after depo- 

 sition in sea-water the initial stages of maturation are already visible. The 

 first indication is a puckering of the nuclear wall on the side nearest the 

 periphery of the egg (figs. 38, 49, 50). Dissolution or rupture of the wall 

 occurs at this point after an interval of from 15 to 20 minutes (time always 

 reckoned from time that eggs are placed in sea-water) allowing an inter- 



