KARYOKINESIS. 23 



The egg nucleus is formed by the fusion of the chromosomal vesicles left in the 

 egg at the close of the second maturation, as described on p. 14. 



The further changes of the germ nuclei may now be briefly followed as far as 

 the prophase of the first cleavage. The developments of both germ nuclei are 

 entirely parallel, so that a single description will serve for both. As soon as the 

 vesicular stage of each nucleus is reached the chromatin is found to be stretched 

 through the nucleus in the form of a reticulum, figs. 36-41. As the nuclei enlarge 

 the chromatin takes more and more the form of rounded masses, Plate III, figs. 44, 

 45 and 46, while the reticulum connecting the masses becomes extremely tenuous 

 and does not stain. In short, there is at first a chromatin reticulum, which in later 

 stages becomes a linin reticulum with the chromatin aggregated at nodal points. The 

 chromatin masses differ considerably in size, fig. 45, and are at first quite solid. In 

 later stages, figs. 49-53, these masses become hollow spherules. Those spherules 

 which develop into chromosomes become connected together into a linear series, 

 and either remain solid or at least have thicker walls than those spherules which 

 take no part in the formation of the chromosomes. The further history of the 

 chromatin will be taken up under the head of the first cleavage. As soon as the 

 vesicular stage of each germ nucleus is reached there appears within it a single large 

 nucleolus. 1 This persists until a stage when the two nuclei come into contact, fig. 

 44, when it is usually dissolved in the nuclear sap, though sometimes traces of the 

 nucleoli may be seen in later stages, e.g., fig. 49. 



3. Egg and Sperm Asters and Spheres. — The history of the egg centrosome 

 and sphere in the second maturation division has already been considered, pp. 16 and 

 17. At the same time that the egg aster is being transformed into the enormous egg 

 sphere, figs. 32-36, a sperm aster has appeared and is undergoing a parallel trans- 

 formation. The various stages in this process occur at approximately the same time 

 in the two, though the sperm sphere and nucleus remain slightly smaller than those 

 of the egg until the nuclei lie near each other. We may now follow in detail the 

 origin of the sperm sphere. 



After its entrance the sperm head lies among the yolk spheres in a small 

 quantity of cytoplasm, while the granules derived from the middle piece lie just 

 behind the head. There is at this stage no trace of astral radiations anywhere in 

 the egg, except in connection with the first maturation spindle. The sperm nucleus 

 lies in this position, near the periphery of the egg, without any trace of astral 

 radiations near it, until the anaphase of the second maturation division. At this 

 time the nucleus has become irregular or amoeboid in shape and some distance 

 from the nucleus, toward the center of the egg, the sperm aster appears. It is a 

 noteworthy fact that no sperm aster appears until the sperm nucleus begins to 

 absorb achromatic material, and this suggests that the two processes stand in some 

 causal relation to each other. Furthermore, the fact that the two spheres are pro- 

 portional in size to their nuclei, and that the sperm sphere remains smaller than 



Mark ('81) observed in an undetermined species of Limax that each of the germ nuclei contained 

 a single nucleolus. 



