DEVELOPMENT OF THE ALBINO RAT 27 



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. While in 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, probabty 

 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 



