1 70 John Hammond, Jr. 



follicle development, and larger numbers of corpora, in the latter group, 

 and to accelerated tubal transport. An association between multiple corpora 

 and degenerate o\a was noted by Brock and Rowson (7) in cows in which 

 estrus was delayed following PMS administration at the time of corpus luteum 

 removal. 



The number of ovulations was greater as the interval between injection and 

 heat increased (7): one may suppose that the longer the PMS has to act, the 

 larger will be the number of follicles mature enough to respond either to an 

 endogenous ovulating release of hormone or to administered PU. 



This factor of timing may have affected the observations of Rowson (71) 

 regarding the influence of preliminary removal of the corpus luteum on the 

 OMjlating effectiveness of PU following PMS treatment. But other data (7) 

 support the finding that, with PMS followed by PU, fewer follicles, and a 

 smaller proportion of those reaching a large size, ovulate if the corpus 

 luteum has been removed. A similar difference (71) existed between the 

 response to whole serum and to processed PMS given at the time of corpus 

 luteum removal (with no subsequent PU). There were fewer ovulations with 

 processed PMS: indeed many cows so treated failed to ovulate within a 

 period of about a week. 



The papers quoted all agree on individual variation in the follicle growth 

 produced, but there are divergent opinions as to whether the general effects 

 to be expected from PMS given at the time of corpus luteum removal are 

 acceleration or delay of heat and ovulation, and ovulation of one or of many 

 follicles. Factors possibly responsible for these differences include not only 

 the different forms in which PMS was given, but also different dose levels, 

 those of Rowson being generally higher. 



Dowling points out that the cow can produce enough hormone to ovulate 

 many artificially stimulated follicles, and scanty data provided by Brock and 

 Rowson suggest that PU given at heat, following PMS given when the corpus 

 luteum was removed, does not increase the number of ovulations. 



The finding of many large follicles and few corpora after a dose of PU which 

 is known to be effective in ovulating a large proportion of a similar number of 

 follicles would appear to indicate that when the PU was given the follicles 

 were either too immature to respond or else were already cystic. 



It seems possible that a small dose of PMS might speed follicle growth 

 after corpus luteum removal, and so, indirectly, hasten ovulation; but a 

 large dose, rapidly absorbed, might provide a premature or subthreshold 

 stimulus to ovulation and thus abort the larger foUicles. Ovulation would 

 then be delayed until a fresh generation of follicles matured. 



AFTER REGRESSION OF THE CORPUS LUTEUM 

 The aims and methods of investigation differ with sheep and cattle and it 

 is therefore convenient to consider the species separately. 



