212 PROCEEDINGS OF THE ACADEMY OF [Feb., 



contains here and there a 3'olk-granule in a vacuole (fig. 2a, y). The 

 nucleus is very large and shows but little stainable chromatin, and 

 that in the form of fine granules on threads of linin. The large nu- 

 cleolus, which stains deeply with orange, contains one or more 

 vacuoles. As in my previous work on the histology of planarians 

 (Stevens, '02), the best results were obtained by fixing the material 

 with sublimate-acetic and staining -^dth Delafield's hsematoxylin and 

 orange. 



The first maturation-spindle is found in the ovary about twenty- 

 four hours before laying. In an eciuatorial stage the spindle is near 

 the centre of the egg. The asters are very large, but there is no evi- 

 dence of centrosome or sphere. The chromosomes are V-shaped, 

 and split longitudinally, giving V-shaped daughter chromosomes, as 

 in figs. 3a and 36. Only four specimens in this stage were obtained 

 out of a large number sectioned; and of these, three had either 3 

 chromosomes in an equatorial plate (figs. 2a and 25) or 6 daughter 

 chromosomes (figs. 3a and 35), and one had 4 in the equatorial plate 

 (fig. 4). Time and material were lacking to trace the egg from the 

 ovary to the uterus, after it was ascertained that an interA-al of about 

 twenty-four hours occurred between the formation of the first and 

 second polar bodies. 



By removing the capsule before the shell is formed and staining with 

 Schneider's aceto-carmine, the second maturation-division can be 

 more advantageously studied than in sections. Figs. 8-11 were made 

 from such preparations; figs. 5a, 55 and 6, from sections. In only 

 two cases was the first polar body observed (figs. 6 and 8), and it seems 

 probable that it is usually lost as the egg passes down the oviduct. 

 The number of egg-chromosomes is 3 in most cases. In two eggs from 

 the same capsule the number was 6 (figs. 9 and 10), and in a few others 

 4 and 5 w^ere observed, indicating that, as in Ascaris mcgalocephala 

 and Echinus 7nicrotuberculatus , there may be two forms which occa- 

 sionally interbreed, one having twice as many chromosomes as the 

 other. 



"J'hus an egg having 6 chromosomes fertilized by a spermatozoon 

 having 3 would give an individual having 9 somatic chromosomes 

 and probably 5 chromosomes in germ-cells after reduction. Union 

 of germ-cells having 3 and 5 chromosomes respectively woiild result in 

 an individual having 8 chromosomes in somatic cells, and 4 in oocytes 

 and spermatozoa. 



Figure 8 shows an egg in which there was no doubt about the number 

 3 in the first polar body (p\), and at the poles of the second maturation- 



