404 BARBARA WIGGENHORN AND DAVID D. WHITNEY. 



As this form of rotifer is oviviviparous and the females fre- 

 quently carry from i-io developing embryos and young it was 

 found practical to fix entire individuals in the killing fluids. 

 Flemming's strong solution for about one hour was used. Then 

 the material was bleached, dehydrated and imbedded in clove oil 

 and paraffin. Large numbers of individuals were sectioned in 

 masses, cutting sections 5 microns in thickness and the favorable 

 sections studied. These animals were raised in great numbers 

 in laboratory cultures in weak horse-manure solutions. The 

 figures were outlined by means of a camera lucida with a magnifi- 

 cation of about 430 diameters and the details filled in free hand. 



As it is the male parthenogenetic egg that can be fertilized 

 and changed into a larger and thicker shelled egg it is of some 

 interest to compare the nuclei of the cells in the embryos devel- 

 oping from these eggs without fertilization and with fertilization. 

 For a complete comparison the female parthenogenetic egg has 

 been included. 



Each cell of the embryo developing from both kinds of the 

 parthenogenetic eggs contains only one nucleus regardless of 

 whether it is in the 2-cell stage or in later stages of a larger 

 number of cells (Fig. i, A-G}. Many hundreds of sectioned 

 embryos have been observed in the various celled stages and not 

 one has been found whose cells contained more than one nucleus. 



A few somatic cells were found in which the chromosomes 

 could be counted fairly accurately in the embryos developing 

 from both kinds of the parthenogenetic eggs. The small male 

 embryos developing from the male parthenogenetic egg showed 

 the haploid number of about 25 and the larger female embryo 

 developing from the female parthenogenetic egg showed the 

 diploid number of about 51 chromosomes (Fig. 2, A-B}. Al- 

 though the chromosomes are very small and somewhat crowded 

 together their exact number is reasonably certain. Tauson 

 working on this same species has concluded the haploid number 

 in the mature male parthenogenetic egg to be 24 and the diploid 

 number of 48 in the female parthenogenetic egg. 



In the early embryos developing from the fertilized eggs two 

 distinct nuclei appear in each cell. These nuclei usually appear 

 apposed to each other but clearly distinct (Fig. i, 77). In later 

 stages only some of the cells sho\v the two nuclei and as the 



