No. 561] 



THE XIX E-B ASHED Ah'M A DI1.U > 



r,33 



pair is derived from one of the first two cleavage blasto- 

 meres, an idea borrowed from the literature on human 

 duplicate twins. Such a theory, however, involves the 

 difficulty of explaining how the cell descendants of one 

 hlastomere would come to occupy a position with refer- 

 ence to one or the other lateral halves of the uterus. The 

 axial orientation of the vesicle is determined by the fact 

 that it always becomes attached by an area of trophoblast 

 at the animal pole, but there is no mechanism for pre- 

 serving a bilateral orientation. There is on the other 

 hand good evidence, as brought out in the last section of 

 this paper, that the definitive bilateral ity of the compound 

 embryonic vesicle is imposed upon it by certain definite 

 bilateral conditions within the uterus, which result in the 

 vesicle being pressed dorso-ventrally and elongated later- 

 ally so as to acquire a bilaterality in conformity with that 

 of the uterus. Thus bilaterality and pairing of fetuses 

 are strictly secondary results and bear no relation to any 

 axes of the egg or planes of cleavage. The closer resem- 

 blances between the individuals of pairs and their closer 

 placental association are due to their common origin from 

 one primary bud, which means that they are genetically 

 more closely related than are the members of opposite 

 pairs. Mirrored-image effects are also made more 

 intelligible by our knowledge of the mode of origin of a 

 pair from a single primary bud, in that when an inherited 

 peculiarity on the right margin of one individual of a pair 

 is found on the left margin of its partner, it means that 

 some median primordium of the primary bud has been 

 split by the secondary budding, so that the resultant char- 

 acter is found repeated on the adjacent sides of the two 



to explain the distribution of many peculiarities inherited 

 by the quadruplets from the parents. 



Sex Ratios and Sex Determination 

 In a collection of 182 sets of fetuses sufficiently ad- 



