244 EMBKYOLOGY OF THE LOWER VEETEBIiATES en. 



pn. 



a.n.d. 



H 



B 



Wet. 



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 

 -os. 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 arctiinephric 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 eliinin- 



133. -Arrangement of archi- ated and the t wo ducts were independent 



from the beginning. 



III. The independently arising 

 Miillerian duct might retain the mode 



Aln:i1 " 17 ' :"-''-.naieimn 1 .;c ) of exten8ioil backwards by intrinsic 



growth, eventually reaching and fusing 

 \s it.h the wall of the cloaca. 



IV. Its separation from t lie somatic 

 mesodt-rm mi-lit take place relatively 



lat.-r than its extension backwards so that it would arise in develop- 

 ment comparat i\<l\ late, as a longitudinal ridge or fold gradually 

 separating oil' from tin; mrsoderm fn>m I -!' m- 



m-phric duct, etc., in embryos of 

 F Pristiwus. (Based on Rabl's 

 figui. 



l--ru;ilt- 27 mm. 



'/, cloaca ; .M.d, Mulli-rnin ilnct ; 



lomjc ojx-iiiux of .Mnlkrian <luct ; pit, 



|)ioin-|.hi-ic ii.-i.liii<stme; W.d, duct ot 



