72O AMNIOTA. 



small rudiment, continuous with the hindermost of the three 

 peritoneal openings, and its solid extremity appears to unite 

 with the walls of the Wolffian duct. 



After the atrophy of the pronephros, the Miillerian duct 

 commences to grow rapidly, and for the first part of its course it 

 appears to be split off as a solid rod from the outer or ventral 

 wall of the Wolffian duct (fig. 406). Into this rod the lumen, 

 present in its front part, subsequently extends. Its mode of 

 development in front is thus precisely similar to that of the 

 Miillerian duct in Elasmobranchii and Amphibia. 



This mode of development only occurs however in the 

 anterior part of the duct. In the posterior part of its course its 

 growing point lies in a bay formed by the outer walls of the 

 Wolffian duct, but does not become definitely attached to that 

 duct. It seems however possible that, although not actually 

 split off from the walls of the Wolrfian duct, it may grow back- 

 wards from cells derived from that duct. 



The Miillerian duct finally reaches the cloaca though it does 

 not in the female for a long time open into it, and in the male 

 never does so. 



The mode of growth of the Miillerian duct in the posterior part of its 

 course will best be understood from the following description quoted from 

 the paper by Sedgwick and myself. 



"A few sections before its termination the Miillerian duct appears as a 

 well-defined oval duct lying in contact with the wall of the Wolffian duct on 

 the one hand and the germinal epithelium on the other. Gradually, however, 

 as we pass backwards, the Miillerian duct dilates ; the external wall of the 

 Wolffian duct adjoining it becomes greatly thickened and pushed in in its 

 middle part, so as almost to touch the opposite wall of the duct, and so form 

 a bay in which the Miillerian duct lies. As soon as the Miillerian duct has 

 come to lie in this bay its walls lose their previous distinctness of outline, 

 and the cells composing them assume a curious vacuolated appearance. No 

 well-defined line of separation can any longer be traced between the walls of 

 the Wolffian duct and those of the Miillerian, but between the two is a 

 narrow clear space traversed by an irregular network of fibres, in some of 

 the meshes of which nuclei are present. 



The Miillerian duct may be traced in this condition for a considerable 

 number of sections, the peculiar features above described becoming more 

 and more marked as its termination is approached. It continues to dilate 

 and attains a maximum size in the section or so before it disappears. A 

 lumen may be observed in it up to its very end, but is usually irregular in 

 outline and frequently traversed by strands of protoplasm. The Miillerian 



