No. 2.| DEVELOPMENT OF THE HUMAN COELOM. 397 
compare the human ovum. Embryologists are accustomed to 
state that the age of a human ovum is to be reckoned from the 
beginning of the first lapsed period, and I think that Bischoff’s 
observation upon the size and growth of the dog’s ovum cor- 
roborates this view. He found that the ova left the ovary 
during the rutting period, but the exact date could never be 
determined. Neither did the time of copulation determine the 
ovulation. Asa rule, it took twenty-four hours or less after 
copulation for the spermatozoa to reach the ovary, and about 
the same time is required for the ovum to reach the beginning 
of the uterine tube after ovulation. So if ovulation and copu- 
lation took place at the same time, fertilization of the ovum 
could not take place until twenty-four hours later. 
In Bischoff’s tables he often rates the age of an ovum from 
the first or from the last copulation, or from the beginning or 
from the end of the rutting period. I have attempted to tabulate 
his specimens from all four of these dates, but in none of the 
attempts did the size of the ova correspond with their respective 
dates. Often eggs of a given date were smaller and developed 
to a less degree than ova presumably younger. After much 
difficulty I finally constructed a table in which the size of the 
ovum and its age correspond. A number of the ova published 
by Bischoff were obtained from the same animal by removing 
half of the uterus at one time and the remaining half the next 
day. In each portion a number of ova were found, and they 
were usually of about the same stage of development. By this 
method of procedure it is possible to determine very accurately 
the growth of the ovum from one stage to one twenty-four 
hours later. So, by gradually plodding through the specimens 
published by Bischoff, it was possible for me to correct his 
data completely. It is remarkable, as the table shows, how 
slowly the development takes place in the early stages, and 
about ten days are required before the ovum is one millimeter 
in diameter. On the fifteenth or sixteenth day the ovum is 
about as large as the human ovum described by Reichert (see 
table). 
Similar results can also be obtained from the various papers 
published on the rabbit’s embryo, Its development, however, 
