jQ Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. 



closely round the archinephric duct. A slight amount of contraction is sufficient to cause the formation 

 of fine chinks between the structures named and it seems a matter of chance whether the duct is left 

 attached to the ectoderm or to the mesoderm or on the other hand is left lying freely between the two 

 layers. No great weight can therefore be given to the fact that the hind end of the duct is attached to 

 one layer or to the other in deciding which layer the duct is originally derived from. There remains the 

 histological character of the duct rudiment and this is clearly mesodermal: the cells forming it being laden 

 with large coarsely granulär yolk. This character indicates that the duct is of mesodermal origin. It is in 

 regard to the further question — whether the hind end of the duct grows back freely by its own growth 

 activity — that the chief difficulty arises. The presence of an occasional mitotic figure in the duct rudiment 

 does not necessarily mean that the duct rudiment is doing anything more than merely keeping pace with 

 the general growth of the tissues. If the duct is growing backwards by its own activity all probability 

 points to the growth activity being localised at its hinder end as it is hardly conceivable that the duct can 

 slide bodily backwards between the cells which closely invest it. That there is no such growth activity 

 at the tip of the duct is indicated by the coarsely granulär character of the yolk contained in it, because 

 the active metabolism associated with active growth is invariably accompanied b}' the breaking down of 

 the yolk into a finely granulär form so as to be readily assimilable. The probability therefore is that the 

 backgrowth of the duct is effected by delamination from the somatic mesoderm. Junction with the cloaca 

 takes place about stage 24 — 25 {Lejiidosiren). 



Up tili about stage 35 {Lepidosiren) the ducts open separately on either side of the cloaca but about 

 this period their cloacal ends are found to be continued backwards into a common portion. About stage 36 

 (Lepidosiren) the region common to the two ducts begins to bulge forwards in front and the projecting part 

 grows actively and forms by stage 37 a long tubulär cloacal caecum. With further growth of the animal 

 the caecum becomes wider and gradually assuines the rounded form characteristic of the adult. 



Mesonephros. 



The mesonephric tubules arise as at first solid rudiments, arranged roughly segmentally and not 

 showing at any period continuity with the myotomes. In Protopterus they begin about segment 14 but in 

 occasional specimens nuclear condensations have been seen in Segments anterior to this (as far forward as 

 the hinder limit of the pronephros) which may possibly represent vestigial tubules. The definitive tubule 

 rudiments become obvious about stage 30. They are at first quite solid and are independent of the duct. 

 Each rudiment assumes a C-shape, a split develops in its interior and its outer end undergoes fusion with 

 the wall of the archinephric duct, the cavity of the tubule and that of the duct soon (stage 31 Lepidosiren) 

 becoming continuous. The free end of the tubule now becomes dilated to form the Malpighian bod}' 

 (stage 32 Lepidosiren) and after a time (stage 35 Lepidosiren) the glomerulus is formed by a pushing in of the 

 wall of the Malpighian cavity. 



Genital ducts. 



The development of the genitorenal connections of the male and of the oviduct in the female has 

 not yet been worked out. In stage 37 {Lepidosiren) the oviducal funnel is present. It passes back into a 

 solid MüUerian duct rudiment which at its hind end appears to die away amongst the mesenchyme. 



Gonads. 



The development of the gonads has not yet been worked out. In stage 38 {Lepidosiren) a cylindrical 

 Strand of gonad with large spherical nuclei rieh in chromatin is clearly visible ventral to the mesonephros. 



