244 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



pn. 



i t 



r r 

 r 



a.n.d. ■- C 



r 

 t 



r 



T 

 r 

 r 



r 

 / 



sr 

 r 

 r 



I 



5 



« 



« 



/ 

 / 

 f 



s 



B 



OS. 



■*Kd 



C 



~3 



Wd 



^0 



^ 



differences are found to exist between the phenomena as described 

 for different groups and even for members of the same group by 



different observers. While some of 

 these may be due to observations being 

 pushed to within the limits of prob- 

 able error it is impossible to avoid the 

 conclusion that great differences do 

 actually exist in the details of develop- 

 ment of the Miillerian duct. 



It is possible on general embryo- 

 logical principles to arrive at an idea of 

 the kind of variations which might 

 be expected to show themselves from 

 the supposedly primitive mode of de- 

 velopment. 



I. The Miillerian duct might con- 

 tinue to arise in an unmodified manner 

 by splitting from the archinephric duct, 

 its funnel being a persisting nephro- 

 stome. 



II. In correlation with the fact 

 that the one derivative of the archi- 

 nephric duct (duct of the opistho- 

 nephros) is required to be functional 

 at a very early period, while the other 

 (Miillerian duct) does not function until 

 adult life, there would be a tendency 

 for the two ducts no longer to keep 

 exactly abreast in their development 

 but to become separated, the Wolffian 

 duct developing relatively earlier, the 

 Miillerian relatively later. To enable 

 this to take place, the primitive stage 

 in which the two ducts were still one 

 would tend to be more and more 

 curtailed until it was eventually elimin- 

 ated and the two ducts were independent 

 from the beginning. 



III. The independently arising 

 Miillerian duct might retain the mode 

 of extension backwards by intrinsic 

 growth, eventually reaching and fusing 

 with the wall of the cloaca. 



IV. Its separation from the somatic 

 mesoderm might take place relatively 



later than its extension backwards so that it would arise in develop- 

 ment comparatively late, as a longitudinal ridge or fold gradually 

 separating off from the mesoderm from before backwards. 



3 





d. 



Fig. 133. — Arrangement of archi- 

 nephric duct, etc., in embryos of 

 Pristiwms. (Based on Rabl's 

 figures. ) 



A, male 17 mm. ; B, female 19 mm. ; C, 

 female 27 mm. a.n.d, archinephric duct ; 

 cl, cloaca; M.d, Miillerian duct; os, coe- 

 lomic opening of Mnllerian duct; pn, 

 pronephric nephrostome ; W, d, duct of 

 opisthonephros. 



