POLYEMBRYONIC DEVELOPMENT IN TATUSIA 629 



settle this question satisfactorily, and that is to study histologi- 

 cally the ovaries of several generations in the family of some 

 animal belonging to a species normally bearing a single young, 

 but showing a tendency to multiple births. If polyovulated fol- 

 licles were present in those individuals which had had multiple 

 births and if the number of corpora present in the ovaries after 

 a given pregnancy were found to be fewer than the number of 

 young in the litter, it would be reasonably certain that the mul- 

 tiple gestations were the result of ova from a compound follicle. 

 Some of the cases in cattle recently cited by Pearl ('12) would 

 have made excellent material for such a test. 



I have discussed the topic of polyovular follicles somewhat in 

 detail in order to emphasize the necessity of keeping it entirely 

 distinct from the subject of polyembryony, for not to do so will 

 most certainly lead to great confusion. This applies not only to 

 the problem of sex heredity, but also to other questions in hered- 

 ity that are capable of elucidation through the study of poly- 

 embryonic development, particularly the one dealing with the 

 limits of hereditary control. It is here essential, as I have already 

 pointed out, that all of the individuals of a litter of embryos to be 

 studied should have the same germinal constitution; but this 

 condition would never be fulfilled in multiple gestations resulting 

 from ova from a polyovular follicle, even if this could be proved 

 beyond question, and even though all the ova came from a single 

 mother cell in oogenesis; because in that event each egg must be 

 fertilized by a different spermatozoon. 



b. Theory of blastotomy. According to this theory each embryo 

 is looked upon as the lineal descendant of one of the early blasto- 

 meres. In the case of two embryos arising from a single egg 

 (identical twins) it has been supposed that each individual is the 

 product of one of the blastomeres of the two-celled stage, while 

 in the case of four embryos (armadillo quadruplets) it has been 

 assumed that each embryo arises from one of the blastomeres of 

 the four-celled stage. And so it has been argued for those cases 

 in which even a greater number than four come from one egg. 



The idea of an early spontaneous blastotomy lying at the basis 

 of polyembryony has been very persistently urged by a number 



