88 EDWARD DRANE CRABB. 



IX. THE CHROMOSOMES. 



The chromosome number has been established for a number 

 of the land gastropods. However, practically nothing definite 

 is known about the chromosome number in hermaphroditic fresh- 

 water snails. Harvey ('20) citing Linville, 1900, lists sixteen 

 chromosomes each for the primary and the secondary oocytes of 

 Lymnoea clodca. The chromosomes of L. s. appressa are very 

 irregular in shape and apparently in number, and this numerical 

 variation appears to be the result of splitting of chromosomes 

 (Figs. 55, 58, 59). 



A. In Egg Cells. 



Because of the great number of irregular yolk granules in the 

 egg cytoplasm which take up the basic stains more readily and 

 retain them more tenaciously than the chromatin itself does at 

 times, it is difficult to distinguish the chromosomes clearly, espe- 

 cially in stages other than the early equatorial. After the ger- 

 minal vesicle breaks down there is no definitely visible egg chro- 

 matin until a number of polymorphic granules appear among the 

 spindle fibers of the archiamphiaster (Figs, n, 12, 13, 16). The 

 various stages in the history of the development of the egg nucleus 

 between the formation of the archiamphiaster and the extrusion 

 of the first polocyte are not sufficiently well known to warrant 

 discussion at this time. However, it appears that late prophase 

 stages are formed as indicated in Figs. 48, 49 and 50. 



a. First Polocyte. Ten chromosomes are readily distinguished 

 in most of the recently extruded first polar bodies, but this num- 

 ber is rapidly increased as the second maturation division of the 

 egg progresses so that by the time the second polocyte is cast out 

 of the egg the first may have fifteen to twenty chromosomes in 

 it. Fig. 53 represents the first polocyte of an egg in which the 

 second polar body is shown forming in Fig. 60. Each of the 

 three first polar bodies represented in Fig. 52-54, as well as the 

 egg nucleus in Fig. 52, contains ten chromosomes. In Fig. 55 

 four of the ten chromosomes have split, thus making the first 

 polocyte seem to have fourteen chromosomes. Likewise the 

 chromosome number of the first polocytes represented by Figs. 56 

 and 57 appears to be about thirteen. The latter was fixed during 



