No. 3.] THE EMBRYOLOGY OF THE EARTHWORM. 395 
in decomposing compost-heaps where the temperature is main- 
tained 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 properties 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 with- 
out following its actual development. In laboratory cultures 
the young worms usually make their escape from the capsule in 
about two or three weeks. 
For the present I am obliged to pass over the phenomena 
relating to the maturation and fecundation of the ova, and my 
account of the cleavage will be found unsatisfactory, owing to 
the impossibility of following continuously the development of 
the individual ova. Development continues for some time 
after removal of the segmenting ova from the capsule, but 
pathological changes invariably supervene, however careful the 
treatment, and I am persuaded that no trustworthy results can 
be obtained by this method. After making numerous 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 cleav- 
age process. In the resting periods the cells are closely pressed 
together, and their outlines are often hard to see; so that it is 
weil-nigh impossible to interpret some of the stages unless they 
are studied in the active period. Moreover, the cleavage process 
varies greatly in the order of division, which after the first two 
divisions loses all appearance of regularity. On account of these 
circumstances the segmenting ova vary widely in appearance, 
and the process of cleavage thus acquires that apparent irreg- 
ularity which other observers have found so perplexing. It is 
now well known, however, that the segmenting ova of various 
other animals (e.g. Mollusca, Coelenterata) are likewise subject 
to considerable variation, which in some cases at any rate is due 
simply to temporary acceleration or retardation in the divisions 
