52 THE MATURATION OF THE EGG OF THE MOUSE. 



Sobotta himself admits in a more recent paper (1907). Although he 

 makes no mention of having seen the beginning of the (large) first spindle, 

 he states (1895, p. 52; 1907, p. 507), without qualification or conclusive 

 evidence, that it originates about 24 hours before ovulation. According 

 to Gerlach (1906, p. 9) the nucleolus disappears completely, and from 

 the chromatin spherules (which he believes owe their origin to the nucleo- 

 lus) the chromosomes are differentiated before the disappearance of the 

 nuclear membrane. Kirkham (19076, p. 73), describing the prophase 

 of the first maturation, says that in a few cases there were traces of the 

 nuclear membrane, though more often it had entirely disappeared. His 

 fig. 1 (plate I), though described as that of an ovarian egg before the 

 formation of the first maturation spindle, looks more like the cross- 

 section of a spindle in the monaster stage than an early stage in the meta- 

 morphosis of the germinative vesicle, and the two detached chromosomes 

 may possibly owe their peculiar position to the displacement which 

 sometimes is caused by the knife in sectioning. 



It will be remembered (p. 25) that the wall of the nucleolus is thick 

 and deeply stained, and that the chromatin bodies of the germinative 

 vesicle are especially numerous around the nucleolus, which lies at one 

 side of the vesicle. Since, in the next stage, the chromosome fundaments 

 (see p. 26) are also at one side of the nucleus, it is probable that they 

 replace both the vesicular nucleolus and the chromatin bodies. This is 

 rendered the more probable by the fact that these fundaments are ar- 

 ranged at one side of a slightly denser part of the nucleoplasm. Such 

 conditions lead one to think it possible that the fundaments arise from 

 both the wall of the nucleolus and the chromatin bodies, while the achro- 

 matic spindle comes from other parts of the nucleus, or possibly originates 

 in the inner part of the nucleolus. 



Precisely how the chromatin of the germinative vesicle is metamor- 

 phosed or differentiated into the fundaments of the chromosomes is 

 unknown ; but in three cases the arrangement of the curved fundaments 

 (as in fig. 36) suggests the possibility that they lie end to end and may 

 therefore be regarded as parts of a potential thread or spireme. This 

 possibility is perhaps strengthened by the fact that these fundaments 

 usually show a longitudinal division first and the transverse division later. 

 These observations suggest that the longitudinal division may corre- 

 spond to the longitudinal split in the spireme of the synapsis stage ob- 

 served in many invertebrates, and that each fundament consists of two 

 univalent chromosomes united end to end. The univalent chromosomes 

 would then be sometimes indicated by the cross-division, and would be 

 separated at the first mitosis, as described on page 30. 



An inspection of the figures of the chromosomes of the first spindle 

 in the papers of Sobotta (1895, ^99, 1907), Gerlach (1906), Lams et 

 Doorme (1907), and Kirkham (19076) reveals the fact that there is no 

 essential disagreement in regard to the general forms of the chromosomes, 



