690 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



determination docs occur in this way, direction of the axis in relation to 

 the uterus as a whole may differ, more or less, according to the region 

 with which the blastocyst becomes most closely associated. If there is a 

 longitudinal or a dorsiventral gradient system in the uterine wall, or a 

 gradient system in relation to each oviduct, the blastocyst adhering to, 

 or implanted in, the mucosa is subjected to the differential in the region 

 concerned. Even in the monotremes axial determination during the in- 

 trauterine period and in relation to intrauterine environment does not 

 appear impossible. ■''* 



Fig. 221, A, B. — Polyembryony in the mnt-handtd. ^xmdiAiWo, Dasy pus novemcinctus. A, 

 outline reconstruction of blastocyst and vesicle with two primary buds, r and / and probable 

 early stage of a secondary bud at s. B, outline reconstruction of later blastocyst and vesicle 

 with four embryonic primordia. // and / supposedly from one primary bud; IV and III 

 from the other (after Patterson, 1913). 



In this connection the case of the nine-banded armadillo {Dasypus 

 novemcindus = Tatusia novemcincta) , in which four embryos develop from 

 a single egg, requires special consideration. Here the ectodermal vesicle 

 formed by invagination of the ectoderm of the embryonic area into the 

 interior of the blastocyst gives rise to two "primary buds," each of which 

 in turn gives rise to two "secondary buds"; and each of these, together 

 with a part of the mesoderm and entoderm, becomes an embryonic pri- 



34 Although no complete search of the literature has been undertaken in attempting to 

 discover evidence for or against the possibility of intrauterine determination of axiate pattern 

 in mammals, it appears that many accounts of early mammalian development do not con- 

 sider the question. 



