Artificial Parthenogenesis in Thalassema Mellita I2I 



acid solution. Since they first become evident in normal eggs 

 about 3 minutes after entrance of the sperm, their appearance is 

 much retarded in the parthenogenetic eggs. They are situated 

 very close to the nuclear wall, and each contains at its center a 

 deeply stained centrosome or centriole (Fig. 17). As soon as the 

 two asters can be discovered at all, they are invariably found some 

 distance apart, and, like Grifiin, I have been able to find no evi- 

 dence that they arise by division of a single aster whose products 

 afterwards diverge. In some cases Griffin {op. cit.,p. 590) observed 

 a bipartite condition of the centrosomes even at this early stage, but 

 in my acid treated eggs I have not found them divided until a 

 somewhat later stage. I have also been unable to discover in eggs 

 which have been exposed to acid solutions any trace of the 

 ''secondary asters" which Griffin (p. 590) has described as being 

 present in both fertilized and unfertilized eggs, but disappearing in 

 the former in about three minutes after entrance of the sperm. The 

 rays of the asters are at first few in number and excessively delicate, 

 but, as they develop, they appear to grow into the nuclear mem- 

 brane, flattening it and throwing it into folds and wrinkles, until 

 they finally rupture it and enter the nuclear area (Fig. 17). As 

 the formation of the amphiaster for the first polar mitosis in nor- 

 mal maturation has been carefully described and figured by Grif- 

 fin, and as these changes are usually the same in every detail in 

 unfertilized eggs which have been subjected to the action of acid 

 solutions, an elaborate account is not necessary here. 



During the continued invasion of the nucleus by the astral 

 rays, the nuclear membrane gradually dissolves away, and the 

 granular reticulum, now staining a dark blue with haematoxylin, 

 is left in the cytoplasm where it afterwards disappears. Some of 

 the rays, which are thicker and stain more deeply than the rest, 

 appear to attach themselves to the chromosomes, the so-called " trac- 

 tion fibers." The centrosomes are now distinctly double, as may 

 be seen in Figs. 18 and 19, which show stages in the formation of the 

 amphiaster. The definitive spindle (Fig. 20) carrying the chromo- 

 somes in the equatorial plate, now swings into a radial position in 

 such a way as to bring the outer aster close to the surface of the egg. 

 The outer rays of this aster shorten, while those situated more 



