6^ Papers from the Marine Biological Laboratory at Tortugas. 



The primary oocyte near the end of the growth-period has a large eccen- 

 trically located nucleus. The nuclear network is wholly achromatic and 

 enmeshes several chromatin nucleoli. The number of nucleoli at this stage 

 (fig. 87) is never less than two and more frequently six or more, all without 

 vacuoles. As development proceeds, the oocytes enlarge slightly in size. 

 There appears now a progressive increase in the amount of chromatin (fig. 

 88). The nuclear network becomes more closely meshed and the threads 

 become stout and intensely chromatic (fig. 89). A single nucleolus only re- 

 mains. A slightly later stage (fig. 90) shows the same process merely carried 

 to a greater degree. The nuclear reticulum, furthermore, appears arranged 

 in the form of a stout spireme. The single nucleolus still persists. 



In a succeding stage (fig. 91) the nucleolus has entirely disappeared. 

 The chromatin network has become arranged in a tangled knot and the 

 astral rays of the first polar spindle are pushing upon the fading nuclear wall. 

 The single ( ?) spireme breaks up into a number of chromosomes (fig. 92), 

 which are drawn as bilobed bodies into the equatorial plane of the first 

 polar spindle (fig. 93). The achromatic parts of the spindle are very coarse 

 and distinct. As in Asterias, here, also, the first maturation spindle is some- 

 what larger than the second. There is no indication of the persistence of 

 the central aster of the second polar spindle about the female pronucleus, as 

 was noted in the case of Asterias. Due to the very minute size of the 

 chromosomes, both in the maturation and cleavage divisions, the exact 

 number could not be counted, nor could the manner of the reduction be defi- 

 nitely determined. The reduced number of the chromosomes lies somewhere 

 between 16 and 20. Some evidence appears here also to indicate that the 

 reduction is accomplished by a double longitudinal division of the original 

 bivalent chromosomes. Figure 93 shows the elements of a postsynaptic 

 chromosome drawing apart, with each element at the beginning of a longi- 

 tudinal fission. The chromosomes in the anaphase of the second polar 

 spindle are still bilobed and so support the hypothesis of a second longitudi- 

 nal division. 



In the case of Hipponoe esculenta it is clear that the chromosomes 

 originated proximately from the nuclear reticulum of the full-grown pri- 

 mary oocyte. It is equally clear that ultimately they received much (pos- 

 sibly all) of the chromatin from the disappearing nucleoli. Hipponoe thus 

 agrees with Asterias in the essential point that the chromosomes do not 

 originate from within the nucleoli, but that the latter contribute chromatin 

 (in Hipponoe much ; in Asterias little) to them prior to their entrance into 

 the first polar spindle. A marked difference in the process between these 

 two forms is in regard to the time when the chromosomes are first formed. 

 In Asterias they appear very early in the growth-period and persist as 

 individual bodies until maturation. In Hipponoe the chromosomes arise 

 from the segmenting chromatin spireme, which makes its appearance only 

 toward the close of the growth-period. There is a general similarity of the 



