IO2 CHARLES V. MORRILL. 



Fig. 7, d, e and /, are from another embryo in the same stage as 

 the last. Fig. 7, g and //, arc from a slightly later blastoderm 

 and i is from a completed blastoderm. 



An inspection of Figs. 6 and 7 as a whole, shows that in the 

 earlier stages, the chromosomes are somewhat elongate (Fig. 6, 

 a g; Fig. 7, a-/) and that as development proceeds, they become 

 shorter and thicker until at the time of invagination or just before 

 (Fig. 6, k-l; Fig. 7, i) they have about the same contour as those 

 of the spermatogonia and oogonia (compare with Fig. I, a-d~). 

 In every stage the tw-chromosomes, though very minute, are 

 constant elements. The unpaired idiochrosome in the male 

 groups and the paired idiochromosomes in the female cannot be 

 distinguished by their size or contour but are probably repre- 

 sented among the larger chromosomes. The remaining chromo- 

 somes cannot be readily paired off. 



2. Anasa tristis. 



The embryonic mitoses of Anasa, though having a larger num- 

 ber of chromosomes that those of Archimerus, are much more 

 favorable for making chromosome counts, especially in the early 

 (incomplete) blastodern stage, i. e., at a time when many of 

 the cleavage nuclei have reached the surface and are still rapidly 

 dividing. The embryos are of tw r o classes: one having 21 and 

 the other 22 chromosomes. Since these numbers correspond to 

 those in the spermatogonia and oogonia respectively, it may be 

 concluded that the 21 -chromosome class are males, the 22-chro- 

 mosome class, females. Fig. 8, a-h, show eight metaphase groups 

 from the 2 1 -chromosome class. Fig. 8, a, b, c, d, e and /, are 

 taken from an embryo in the early blastoderm stage. In this 

 embryo ten more perfectly clear groups were found each with 21 

 chromosomes, making sixteen in all from the same embryo. Fig. 

 8, g and h, are from another embryo in the same stage. In Fig. 

 9 six groups of the 22-chromosome class are shown, all from the 

 same embryo in the early blastoderm stage. 



One exceptional group was found in an embryo of the 22- 

 chromosome class (Fig. 8, i). This group contains 23 chromo- 

 somes, of which three arc larger than the rest. It is difficult to 

 suggest an explanation for this condition. It may be due- to an 



