1890.] Microscopy. 1101 
deposited in earth, usually a few inches below the surface. Those of the 
first species are irregularly fusiform, and of a brighter yellow color than 
those of Z. fetidus ; they measure on the average about 5-7 by 3-5 
millimeters, Those of Z. Zerrestris are still larger (mean measure- 
ments are 6-8 by 4-6 mm.), regularly fusiform, but more swollen and 
rounded than those of the other species; their color is a dark olive, 
In both species the albumen has a slimy, mucus-like consistency, and 
is not greatly hardened by reagents. In Z. terrestris only one egg 
develops out of several included in the capsule, Jn Z. communis two 
embryos are produced as a rule, and in many cases, though not in all, 
both arise as twins from a single ovum, as has been described by Kleinen- 
berg. 
Egg-laying seems in special cases to continue throughout the year, 
though it is most active in the spring and summer months, I have 
found the capsules of Z. fetidus out of doors in nearly every month of 
_ the year, but in mid-winter they are only found in decomposing com- 
post heaps where the temperature is maintained at a tolerably high 
point, The rate of development varies greatly, and depends not only 
upon the temperature, but also upon the vigor or other internal prop- 
erties of the individual embryos, for in late stages the embryos in a 
single capsule are often found in very different stages of advancement. 
It is therefore impossible to determine the age of the embryo without 
following its actual development. In laboratory cultures the young 
worms usually make their escape from the capsule in about two or three 
weeks, 
Development continues for some time after removal of the segment- 
ing ova from the capsule, but pathological changes invariably super- 
vene, however careful the treatment, and Iam persuaded that no trust- 
worthy results can be obtained by this method. After making numer- 
ous drawings of embryos thus studied, I rejected them all, and relied 
wholly on the comparative study of specimens examined or preserved 
immediately after opening the capsules. The results thus obtained, 
though based on the examination of a very large number of specimens, 
are necessarily incomplete; but I believe them to be trustworthy 
as far as they go. 
As in so many other cases, periods of quiescence, or ''resting _ 
stages,’’ alternate with periods of division throughout the cleavage 
process. In the resting periods the cells are closely pressed together, 
and their outlines are often hard to see; so that it is well-nigh im- 
possible to interpret some of the stages unless they are studied in the 
active period. Moreover, the cleavage process varies greatly in the 
