hall: mesonephros and mulleuian duct in AMrniBIA. 87 



(Fig. 49, PI. 4, tae. e'th. y') along tlie line where the body-wall and 

 shelf join each other. As tlie second evagination — which I shall hence- 

 forth call the odium — has migrated c~Jindad until it is well out of the 

 glomerular cavity, the thickened band no longer needs to make a bend 

 cephalad to reach it. Instead it runs straight back, becomes a groove, 

 then by further folding a tube, and finally continues caudad as the 

 Miillerian duct, which is now simple and shows no signs of its double 

 origin (see Figure N, where, however, it is represented as double). 

 Figui-e 52, tae. eHli. y', represents the band after it has passed the pos- 

 terior limit of the shelf and a short distance anterior to the ostium.^ 

 On the left side of the body the anterior evagination and a portion of 

 its duct ai'e retained in a remarkable degree of perfection. Figure 49, 

 evg. 1, shows in section this first evagination, which has lost all con- 

 nection with the thickened band, as shown in Figure N. Its duct is 

 represented by a continuous cord, the nuclei of which are arranged radi- 

 ally, which disappears, however, before it reaches the ostium (or second 

 evagination). On the right side both the first evagination and its duct 

 have disappeared, witii the exception of a portion of the latter, which, 

 though only a few sections in length, is well formed (Fig. 52, df. 1). The 

 retention of this detached bit of the anterior duct is important, as it 

 lends strength to the assumption made in the description of Larva YII 

 (page 82), that the detached portion there seen was a survival from the 

 degenerating duct of the first evagination. 



The grooved condition of the thickened baud anterior to the ostium 

 is especially noteworthy. It signifies that the process whereby the 

 ostium is transferred to a more and more anterior position has already 

 begun. This process consists in a longitudinal folding of the thickened 

 band, followed by a fusion of the edges of the groove thus formed, 

 ■which, beginning at the ostium, advances cephalad. 



Larva XII, 55 mm. — Neither gills nor gill-slits. 



This is the latest stage which I shall describe. It is essentially like 

 the preceding, with the exception that all traces of the anterior evagi- 

 nation and its duct have disappeared. The glomerular cavity is much 

 reduced in diameter. It has the form of a long naiTow^ tube (Fig. 45, 

 Plate 4, cav. glm.) nearly filled at its anterior end with the now thread- 

 like glomus. It still connects with the sub-glomerular cavity (now 

 synonymous with the body cavity, the pneumo-parietal fusion having 



1 At earlier stages, this portion of the band was designated as y3. 



