322 CELLS, TISSUES, AND ORGANISMS 



phoblastic activity ceases and nutrition is derived by diffusion from the 

 maternal blood stream. 



In the blastocyst stage genetic differences in size between differ- 

 ent breeds of rabbits have been observed by Gregory and Castle (1931). 

 At a given stage the large breeds have a larger number of blastomeres. 

 Both between species and within a species the amount of some sub- 

 stance or substances in the uterine secretions, possibly in the nature of 

 a growth hormone ( Hammond, 1959 ) , limits the number of blastocysts 

 that can develop. For example, if in the ewe, which normally produces 

 a maximum of about three lambs, 30 fertilized eggs (Figure 1) are 

 produced as a result of the injection of pregnant mares' serum, all but 

 about three perish in the blastocyst stage ( Robinson, 1951 ) . The ferti- 

 lized eggs can, however, develop normally if they are transplanted two 

 at a time into other ewes ( Rowson and Adams, 1957 ) . 



In the embryonic stage, differentiation of the organs and tissues 

 occurs by cell multiplication, and the nutrition for this is provided by 

 the activity of the trophoblast, the cells of which, like cancer cells, have 

 a high priority for nutrients from the blood stream. As a result, the size 

 of the young animal at this stage is not affected by the level of nutrition 



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Figure 1. Survival of fertilized ova after normal ovulation and superovula- 

 tion. In spite of the presence of as many as 30 fertilized ova, all but an 

 average of two or three perished. (Robinson, 1951.) 



