690 EMILY RAT GREGORY, 



of the pronephric region it has closed and the tubule is entirely cut 

 off from the coelom. This fact will be considered again under the 

 discussion of the mesonephros. There are, however, interesting ex- 

 ceptions to this fusion of pronephric and mesonephric elements and 

 to the median position of the posterior funnels. First, in some em- 

 bryos we find tubules with two dietinct funnels, sometimes widely 

 separated, evidently the pro- and mesonephric funnels, with but one 

 opening into the duct and in one case, at least, the tubules remain 

 separate although extending over almost the same sections, see embryo 

 III (5), somite VII, sections 3—10 (Plate 46, Figs. 40—47). Again, 

 where the funnel is single we find sometimes, notably in embryo VI (2), 

 the tubule extending on one side to the duct and on the other towards 

 the aorta, where we see clearly the widening of the end of the tubule 

 to receive the glomerulus, at this time unformed or represented by 

 irregular tufts of very typical cells. 



Here, too, we find branches from the aorta running to the tubules 

 and the formation of a glomus. The aortic branches start from the 

 lower part of the aorta on each side. In two apparently normal 

 embryos, III (4) and III (5), several of them are opposite each other, 

 but in no case do the branches of the two sides correspond exactly, 

 either in size or number. In III (4) there are nine on the right and 

 five on the left. In Embryo III, 5, somite VI, 7, there may be seen 

 two pocket-like evaginations from the aorta, opposite each other. 

 Anterior to this there is but one tubule on the left side, but beyond 

 this point they are on both sides. The aortic branches can be seen 

 again in VII, 5, 6, 7 (Plate 46, Figs. 42—44), and in VIII, 13. In 

 all there are seven branches on the right and nine on the left. The 

 lacunae here are rather small, and may be seen in sections VI, 6, 

 VIII, 3, and others. In both cases III (5) and III (4) the first 

 aortic branch is in the sixth somite. In one, they end in the ninth, 

 and in the other, in the tenth somite. In Embryo V, 2 the lacunae 

 are much more conspicuous, noticeably in somites VIII and IX, see 

 Plate 46, Figs. 31 and 32; and other series show them even more 

 strikingly. Another embryo IV (1), also apparently normal but more 

 advanced than the former, has fifteen aortic branches on the left, and 

 only eight on the right, beginning in the fifth and ending in the 

 tenth somite. Still another, IV (5), is very much behind in this re- 

 spect. It has only one aortic branch on the right and five on the 

 left, with the first point-like beginnings of others. These aortic branches, 

 which begin as pointed or pocket-like evaginations from the aorta, 



