FERTILIZATION AND CLEAVAGE 91 



responding adult weights in the small race (B) are 1500 grams 

 for females and 1400 grams for males. The various hybrid 

 combinations show roughly intermediate adult weights. 

 Their data show clearly that certainly beyond the 32nd hour 

 after copulation the cleavage rate is fastest in the large 

 race animals and the expected sort of intermediate rates 

 occurs in the various hybrid combinations. It is entirely 

 possible that even the earliest cleavages do actually occur 

 sooner in large race animals since large does ovulate later 

 than small does and therefore their ova should be fertilized 

 later. The number of mitoses in cleaving eggs of the large 

 races also exceeds those in the small race, as the data in the 

 columns labelled ''prospective" indicate. Since this differ- 

 ence is consistently present in reciprocal hybrids between 

 the races the implication is that the sperm nuclei also 

 participate in the control of the cleavage rate. 



In spite of the inherent differences in the speed of segmen- 

 tation the processes of differentiation occur at the same 

 time in the large and small size rabbits. Thus the blast o- 



Plate VII. All photographs on this plate were made from the living rabbit 

 eggs in Locke's solution, as soon as possible after removal from oviduct or 

 uterus at an enlargement of 180 diameters (apochromatic objective 16 mm., 

 compensating ocular 8). They are arranged in order of development and 

 show the principal features of cleavage and formation of segmentation cavity. 

 It will be noted that the trophoblast is precocious in its differentiation as com- 

 pared with the remainder of the egg, and as soon as the trophoblast becomes 

 histologically different one sees fluid begin to accumulate within the egg, 

 thereby forming the segmentation cavity. 



Figs. 4 to 9, Litter C 43, 25 hours after coitus. Fig. 4, one-cell stage with two 

 polar bodies; Fig. 5, one cell, with coarse granules, perhaps abnormal; Figs. G to 

 9, showing two primary blastomeres, one tending to be larger than other. Figs. 10 

 and 11, Litter C 36, 28^:^ hours after coitus. Four-cell stage with crossed arrange- 

 ment of blastomeres. Figs. 12 to 14, Litter C 45, 32 hours after coitus. 5, 6 and 

 8-cell stages. In Fig. 13 the cell at top is just dividing. Fig. 15, Litter C 35. 16-cell 

 stage. Fig. 16, Litter C 41, 55 hours. Morula of about 32 cells. Fig. 17, Litter C 32, 

 Q9% hours. Smooth surfaced morula. Fig. 18, Litter C 38, 71^ hours. Differen- 

 tiated trophoblast cells on surface. Fig. 19, Litter C 33, 76^ hours. Fluid beginning 

 to collect in cleft between trophoblast and inner-cell mass. At this time the albumen 

 coat is at its maximum. Fig. 20, Litter C 33, 76^ hours. Subtrophoblastic lakelets 

 of fluid determining early appearance of segmentation cavity. Fig. 21, Litter C 34, 

 90 hours. Definite segmentation cavity. Note demarcation between trophoblast and 

 inner-cell mass. Fig. 22, Litter C 42, 92 hours. Zona much stretched and layer of 

 albumen much thinned out. Inner-cell mass flattening into typical germ-disc. 

 From Gregory, 1930. 



