HALL : MESONEPHROS AND MULLERIAN DUCT IN AMPHIBIA. 53 



stripped that of their secondary companions (Fig. 25,fnd. ms'nph. 2), — 

 since the latter remain almost unaltered for a considerable period dur- 

 ing the metamorphosis of the primary units.^ 



All that has been said in regard to the origin of the secondary units 

 applies to that of the tertiary, with the exception that the latter arise 

 by a splitting of, or proliferation from, the secondary blastulae, instead 

 of from the primary. In Figure 27 one of these tertiary blastulae is 

 represented in the process of splitting off from a secondary one. The 

 section figured is especially significant from the fact that a dividing cell 

 is seen between the secondary and tertiary blastulae. 



The tertiary blastula, in its turn, remains unaltered for some time 

 and then gives rise to a fourth blastula. As one passes caudad, the 

 first tertiary fundament appears about half a somite behind the first 

 secondary ; the first quaternary a short distance behind the first tertiary, 

 and so on (see Diagram 1, page 56, column S). 



Later Development of the Dorsal Sets of Units. 



Figure 25 is drawn from a section anterior to the region of the tertiary 

 units. The primary unit presents the form of a sigmoid curve. The 

 secondary blastula {fnd. ms'nph. 2) always lies against this curve at 

 the upper limit of its lateral arm, which I shall call the collecting trunk 

 (trn. clg.). The secondary blastulae which were described as lying 

 dorsal to, and in contact with, the primary blastula have evidently 

 simply retained that position, whereas those which were median to the 

 primary blastula must have moved upward and outward to their present 

 position. This change of position might be brought about by the growth 

 of the Malpighian body of the primary unit, which would tend to force 

 the secondary blastula upward and outward. In the development of 

 the primary unit, the collecting trunk increases in length without be- 

 coming coiled, — the great mass of the unit being formed by the length- 

 ening and coiling of the median arm of the curve {thl. ms'nph., Fig. 25). 

 This fact allows the secondary blastula to retain undisturbed its position 

 against the collecting trunk at the periphery of the mass. 



With the increasing complexity of the primary unit, its Malpighian 

 body and outer funnel, as well as the "Wolffian duct, are shoved ventro- 

 laterad until they lie at some distance from the mesentery (Fig. 24). 



1 The early appearance of the secondary fundaments was a surprise to me, as all 

 previous autliors have stated that they are first visible vphen the primary units are 

 far along in their development. 



