DEVELOPMENT OF THE ALBINO RAT 109 



insemination. The resulting 2-cell stage extends for a period 

 of about 24 hours and is found in about the middle of the oviduct. 

 The first two blastomeres are equivalent cells. One of these 

 segments before the other, resulting in a 3-cell stage, present 

 for each ovum for only a relatively short period. 



The 4-cell stage is observed at the end of the third day after 

 insemination. The ova have by this time traversed about nine- 

 tenths of the length of the oviduct. 



The 8-cell stage is observed the latter half of the fourth day 

 after insemination and at the end of the fourth day the ova 

 pass from the oviduct to the uterus in the 12-cell to 16-cell stage. 

 The oolemma is lost usually in the 4-cell stage, the segmenting 

 ova conforming in shape to the general form of that portion of 

 the oviduct in which they are found. 



Three successive segmentation stages, spaced at intervals 

 of about 18 hours, resulting in 2-, 4-, and 8-cell stages occur dur- 

 ing transit through the oviduct. During the fourth segmenta- 

 tion the ova pass from the oviducts to the uterine horns, at the 

 end of the fourth day. 



The mass increase of the ova during the first three segmenta- 

 tions is approximately from 0.15 c.mm. in the pronuclear stage 

 to 0.18 c.mm. in the 8-cell stage. The slow rate of segmentation 

 and the relatively small mass increase may be attributed to the 

 relative scarcity of the embryotroph during transit through 

 the oviducts. 



During the early hours of the fifth day after insemination, all 

 of the segmenting ova are found lying free in the lumen of the 

 uterus, spaced about as in the later stages of development, the 

 fifth series of segmentations having been completed by this 

 time, the resulting morula masses having ovoid form, measuring 

 approximately 80 /i by 50 /i and consisting of from 24 to 32 cells. 

 The mechanism operative in spacing the ova in the uterine horns 

 has not been determined. 



The early stages of blastodermic vesicle formation are observed 

 during the middle and latter half of the fifth day. The seg- 

 mentation cavity begins as a single, irregularly crescentic space, 

 eccentric in position, and arising between the cells of the morula. 



