DEVELOPMENT OF THE ALBINO RAT 41 



thus the beginning of the fourth day, in a stage in which L6 cells 

 up to 32 cells may be enumerated; the oolemma having been 

 lost in the 8-cell stage. The data furnished by Melissinos as 

 concerns the mouse, are as follows: The 2-cell stage is obtained 

 at the end of 24 hours after copulation, the 6-cell stage dming 

 the first 12 hours of the second day, and the 28-cell stage during 

 the second 12 hours of the second day. The ovum is said to 

 pass into the uterine horn at the end of the third day after cop- 

 ulation, retaining its oolemma. The account of Sobotta seems 

 the more reliable. Hensen describes a 2-cell s< age in the guinea- 

 pig 22 to 24 hours after copulation, and BischofT records that the 

 ovum of the guinea-pig passes into the uterine horn while in the 

 8-cell to 16-cell stage, toward the end of the third day. Heape, 

 who has described very fully the segmentation stages of the 

 mole (Talpa europea) gives no data as to the rate of segmenta- 

 tion. In the explanation of the figures presented it may be noted 

 that the ova figured, showing 2-cell to 15-cell stages, were taken 

 from the oviduct. His figure 20, showing an ovum 'fully seg- 

 mented' was Obtained from the anterior end of the uterus. Asshe- 

 ton gives for the rabbit the following data: The 2-cell stage is 

 obtained about 24 hours and the 4-cell stage about 26 hours 

 after coitus. The third series of divisions begins about 28 hours 

 after coitus, so that by the end of the second day a typical morula 

 of 16 cells to 20 cells is to be found. Between 73 hours and 96 

 hours the beginning of the blastodermic vesicle formation is to 

 be noted. Ova obtained 80 hours after coitus, still surrounded 

 by the oolemma, were removed from the uterine horn. Data 

 as to the relative position of the ova in the oviduct in the several 

 stages of development discussed, are given. As concerns the 

 sheep, Assheton states that the ova pass into the uterine 1 horn 

 early on the third day after mating. The pronuclear stage i- 

 to be observed the second day, and the first segmentation at the 

 end of the second day. By the fourth day, with the ova in the 

 8-cell stage, they are found in the upper end of the uterine horn. 

 The blastodermic vesicle formation begins with the 16-cell stage. 

 Again, according to Assheton, the ova of the pig pass to the uterus 

 about the third day after fertilization, if I read him rightly, 

 reaching the uterus in the 4-cell stage, although ova in the 2-cell 



