BIOLOGY OF EGGS AND IMPLANTATION 



853 



this difference throughout the period of cul- 

 tivation. 



The successful transplantation of eggs 

 from animal to animal in certain rodents is 

 feasible and may be the means whereby 

 an experimental approach to the problem of 

 spacing can be made. One or more fertilized 

 eggs could be transferred to the oviducts of 

 properly timed hosts and their sites of at- 

 tachment observed. One of the problems in 

 evaluating implantation grossly in trans- 

 plantation e.xperiments is the possibility of 

 inert objects (lint, clumps of cells, etc.) 

 affecting the decidual response and mimick- 

 ing imiilantation. 



In normal, pregnant rats the embryos are 

 more evenly spaced in cornu when the num- 

 ber of young is 5 or more. If the number of 

 implanting blastocysts is less than 4, there 

 is a tendency to occupy chiefly the caudal 

 halves of the horns (Frazer, 1955). 



Information is needed as to the manner in 

 which eggs enter the cornua, i.e., whether 

 they enter singly or as a group and what 

 the relationship of the multiple eggs may be 

 one to another during the several days that 

 they lie free within the uterine lumen. It is 

 ciuite clear that embryonic spacing in utero 

 is more even than random. This raises the 

 cjuestion as to what controls the size of the 

 refractory area if the cornu is crowded by 

 superovulation, transplantation of eggs, or 

 more than normal numbers of eggs from 

 compensatory hypertrophy in cases where 

 one ovary has been removed. 



It has long been known that in bicornu- 

 ate uteri blastocysts may pass from one 

 cornu into the other through the body of 

 the uterus (Boyd, Hamilton and Ham- 

 mond, 1944; Boyd and Hamilton, 1952; and 

 many others). Bischoft' (1845) interpreted 

 transuterine migration as a method by 

 which the distribution of embiyos could be 

 equalized in cases where there is a disparity 

 in the number of eggs ovulated from each 

 ovary. The means by which this migration 

 is accomplished has been the subject of 

 speculation and some investigation. 



At present, there is no direct evidence that 

 the unimplanted embryo has the power of 

 independent movement. If this is true then 

 the positioning of the blastocyst in utero 

 and its orientation in relation to the endo- 



metrium must depend on chemical and/or 

 physical forces. Markee and Hinsey (1933) 

 suggested that alternate contractions of the 

 cornu transport blastocysts from one to an- 

 other. Krehbiel (1946) anastomosed the 

 cornua of ovariectomized rats in a variety 

 of ways and concluded that each uterine 

 cornua retains its individuality in effecting: 

 the distribution of embryos. 



The role of the myometrium in the distri- 

 bution and spacing of the blastocysts in 

 utero has received considerable attention. 

 Corner (1923) and Wislocki and Gutt- 

 macher (1924) found active myometrial 

 contractions in the sow during the pre- 

 implantation period. Even though the 

 postovulatory contractions occurred with 

 greater frequency, they were greatly dimin- 

 ished in amplitude compared with those re- 

 corded during the estrous phase. The motil- 

 ity pattern of the myometrium changes 

 gradually from day to day so that, by the 

 time of implantation (12th or 13th day), 

 the spontaneous contractions continue at a 

 rate of 4 to 8 per minute, but their ampli- 

 tude is so slight that the kymographic trac- 

 ings are almost level. Similar observations 

 were reported for the excised uterine horns 

 of the rabbit (Knaus, 1927). Using a more 

 refined technique and beginning their ob- 

 servations immediately after the muscle 

 strips were put into the bath, Csapo and 

 Corner (1951) and Csapo (1955) showed 

 that uterine muscle under the dominance of 

 progesterone displays a high state of irrita- 

 bility but poor conduction, and it develops 

 spontaneously a state of "contracture" 

 when it is first placed in the muscle bath. 

 Spontaneous contractions begin after a short 

 interval but they are of very low amplitude. 

 The initial "contracture" is reversible and 

 may be suspended by electrical stimulation 

 or anoxia. Progesterone in some way alters 

 the response of the myometrium to stimuli. 



The motility pattern of the myometrium 

 under the dominance of progesterone is cer- 

 tainly different from that when the animal 

 is in estrus, but the nature of these differ- 

 ences is still puzzling (Reynolds, 1949; 

 Csapo and Goodall, 1954). A strip from an 

 estrous uterus placed in the bath relaxes 

 immediately. After a short interval, spon- 

 taneous contractions begin and continue 



