DEVELOPMENT OF THE ALBINO RAT - 2k 
end of the tubes. In six of these, the ova are quite closely 
grouped as given in the reconstructions shown in figures 9 and 
10. In the remaining two they were more widely spaced, about 
as shown in figures 7 and 8. In the oviducts taken from rats 
Nos. 58, 61, 62, killed respectively 2 days, 17 hours, 2 days, 18 
hours, and 2 days, 22 hours, after insemination, the ova are 
found in a portion of the tube which corresponds very closely 
to that shown in the reconstruction presented in figure 9. In 
rat No. 60, killed 1 day, 18 hours after insemination, the ova 
are more widely spaced and are situated in a segment of the 
oviduct approximately one loop nearer the fimbriated end than 
that given in figure 7, a model of the oviduct of rat No. 59, killed 
two days after insemination. 
In one of the segmented ova of rat No. 60, the two blastomeres 
resulting from the first segmentation division are distinctly sep- 
arated by a space equal to about one-half of the diameter of each 
of the cells. No oolemma is discernible. The two separated 
cells appear normal in size, shape, and structure, as do also their 
nuclei. They lie free in a slightly distended portion of the lumen, 
and appear not to have been separated as a consequence of ma- 
nipulation. The possibility of each developing separately is 
suggested, and may be offered as a possible explanation of the 
occurrence of very small embryos now and then found among 
others showing normal development. King states that “On 
dissecting pregnant females (rats) one frequently finds one or 
more embryos that are much smaller than the rest. Whilein some 
instances such small embryos appear normal and are presumably 
either runts or embryos that have resulted from superfecunda- 
tion, in the majority of cases they are pathological, probably 
because of faulty implantation of the ovum.’ My own material 
contains pathologic ova and embryos in different stages of 
development. This portion of the material will be considered 
in Part II, where the possibility of the occurrence of half em- 
bryos will be discussed. 
As may have been seen, the 2-cell stage of the albino rat covers 
a period of somewhat more than 24 hours, extending from about 
the middle of the second day until toward the end of the third 
