124 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 



subsequent development of a perinuclear zone, as described above. 

 The nucleus then commences to grow and faint radiations can some- 

 times be seen extending from the perinuclear zone into the surrounding 

 cytoplasm. 



During immersion in the hypertonic solution there are no apparent 

 changes beyond a slight reduction of the clear zone of hyaloplasm 

 surrounding the nucleus. 



After the eggs are put back into normal sea-water the internal 

 changes resulting in the first cleavage follow each other in quick suc- 

 cession. The first change noticed is an increase in the development of 

 the perinuclear zone, followed by further growth of the nucleus. Mean- 

 while, the meshwork of chromatin becomes coarser and more aggre- 

 gated together and the nucleolus gradually disappears. This stage 

 is succeeded by a reduction of the perinuclear zone together with its 

 radiations. 



About half an hour after transference to normal sea-water, from 

 one pole of the nucleus a definite aster begins to develop, its rays 

 focusing in a more or less indistinct centrosome situated on the nuclear 

 membrane. 1 By division of the centrosome a typical amphiaster is 

 formed in the nuclear area and as it develops the nuclear membrane 

 disappears. At the same time the chromatin assumes the form of 

 a spireme, which subsequently breaks up into about 18 long and 

 slender chromosomes. At this stage it is impossible to clearly dis- 

 tinguish their number, but, as the chromosomes are gradually drawn 

 into the equator of the cleavage amphiaster, they shorten consider- 

 ably and become quite distinct by the time that the equatorial plate 

 is formed. 



At this stage we have made numerous counts of the chromosomes 

 and invariably found it in the neighborhood of 18, which is half the 

 number that is normally present in this species. 



This behavior is very similar to the one found in the egg. 

 after fertilization by sperm. 



1 It may be well to call special attention to the fact tliac the centrosomes and 

 astrospheres are not formed while the eggs are in the hypertonic solution but 

 some time after they are put back into the normal sea-water. Only if the eggs 

 remain too long in the hypertonic sea- water centrosomes and cy tasters may form 

 while the eggs are in the hypertonic sea-water, not, however because they are in 

 this solution but in spite of it. The hypertonic solution allows the internal process 

 leading to the formation of astrospheres to go on for some time. This led to the 

 erroneous idea that the hypertonic solution was the direct cause of the formation 

 of centrosomes and astrospheres. 



