Ch. I] FORMATION OF THE SEX-CELLS 13 



fereiitly from the rest, and are spoken of as the yolk-nuclei. 

 They seem, in some way not fully understood, to be con- 

 nected with the development of the yolk-granules.^ The 

 yolk-granules, at first small and scattered, grow larger and 

 become more numerous (Fig. 8). Before the egg leaves the 

 ovary, the nucleus wanders toward the periphery and places 

 itself under the black pole of the egg 

 (Fig. 9). When the surface of the 

 egg is examined, it shows a lighter 

 area, owing to the displacement of the 

 pigment-granules in the region occu- 

 pied by the nucleus. The nucleoli at 

 this time migrate toward tlie centre of 

 the nucleus,^ there disintegrate, and 

 finally disappear. The chromatin-mate- 

 rial draws together at this time into Fig. 9. — Section of ripe ova- 



, , , . , 1 1 ii rian egg. (After Hertwig.) 



threads which stain more deeply, the 



nuclear membrane disappears, an achromatic spindle develops, 



and the egg is ready to extrude the first polar body. 



Comparison of Spermatogenesis with Oogenesis 



The method of extrusion of the polar bodies is described in 

 the next chapter, but we may anticipate this account in order 

 to consider here a remarkable parallel that has been discovered 

 between the formation of the polar bodies and the formation of 

 the spermatozoa. In the latter, as we have seen, two successive 

 divisions follow each other during the maturation-period tvith- 

 out an mtervening re sting -stage. The tetrad-groups are present 

 at the beginning of the process. After the two maturation- 

 divisions the number of chromosomes is reduced to half the 

 number characteristic for the species.^ The same phenomena 

 appear when the polar bodies are extruded from the egg. After 

 the extrusion of the first polar body, the spindle for the second 



1 Will ('84) describes the yolk-nuclei as arising from constrictions of the 

 nucleus set free with their nucleoli into the protoplasm. 



2 Schultze aifirms that the later chromatin comes from these nucleoli, but 

 Born has corrected this statement. 



3 The reduction in number of the chromosomes seems in some forms to take 

 place before the tetrad-period. 



