ORIGINS OF EMBRYONIC PATTERNS 691 



mordium. The two primary buds arise on right and left sides of the 

 vesicle (Fig. 221, ^), facing the openings of the Fallopian tubes (Patterson, 

 1912, 1913). That factors in the uterine environment are concerned in 

 localizing the two primary buds and in orienting their opposed polarities 

 appears more probable than that they are determined independently of 

 environment by fissions of the blastocyst (Newman, 191 7, pp. 47-49)- 

 According to Patterson, two thickenings appear on each primary bud, 

 one at its tip, the other on its left side {s of Fig. 221, yl), as viewed from 

 the center of the free surface of the blastocyst. Later the four embryonic 

 primordia appear more or less equidistant from each other (Fig. 221, B), 

 two of them, // and IV, retaining the positions of the primary buds, the 

 others, / and ///, being respectively dorsal and ventral. How formation 

 of a secondary bud on the left side of each primary bud is determined re- 

 mains a problem; if it indicates an asymmetry in the primary bud, the 

 origin of that is entirely obscure. 



In another species of armadillo^-^ tertiary buds arise in variable num- 

 ber, six to twelve embryos being formed (Fernandez, 1909). It has been 

 suggested that a number of radially arranged apical points are determined 

 in the ectodermal vesicle before visible budding and that those which 

 happen to lie toward right and left sides of the uterus have more room for 

 growth (Newman, 1917, pp. 50-51). This hypothesis is without support 

 of evidence and does not seem to be required by the data of observation. 

 Differences in rate and somewhat less uniformity in physiological domi- 

 nance and isolation would permit repetition of budding more frequently 

 in some of the primary or secondary buds than in others. Moreover, it is 

 difficult to conceive how such a radial system of presumptive embryonic 

 primordia could arise all at once from what Newman regards as origi- 

 nally a single embryo. The budding of the ectodermal vesicle results in 

 the determination of a new longitudinal axis for each embryo and, conse- 

 quently, a new bilaterahty (Fig. 221, 5); but dorsiventrality remains the 

 same in all. 



VERTEBRATE ASYMMETRIES 



All vertebrates are normally asymmetrical as regards position of vari- 

 ous organs — heart, stomach, liver, coiling of intestine, etc. — and exhibit 

 various functional asymmetries. In general, these asymmetries are highly 

 constant as regards laterality, but situs inversus occurs occasionally. 

 That genetic factors are concerned in some way is evident, but how the 

 constancy of laterality is determined is no better understood here than 



i'^ Das y pus hyhridiis=Taliisia hybrida = Mulila hybrida. 



