The development of the urinogenital organs of the lamprey. (37 



the following manner (p. 502): "Ich sehe daher in dem Glomus eine 

 secuudäre, erst mit der Umformung der Vorniere eintretende Bildung, 

 die sich dann in der schnell vorübergehenden Entwicklungsgeschichte 

 anderer Vornieren als letzter Rest des segmentalen Gefässnetzes noch 

 erhalten und zur bisherigen Definition der Vorniere Anlass gegeben 

 hat." And he goes on to sum up the distinctions between the pro- 

 and mesonephros as follows^): "Als charakteristisch für eine Vor- 

 niere im ursprünglichen Sinn ist danach nicht ein Gefässknäuel und 

 ihm gegenüber liegende Trichtercanälchen im Cölom anzusehen, sondern 

 einzelne segmentale Canälchen, die im Cölom beginnen, von je einem 

 lacunären Gefässnetz begleitet werden und die zuerst einzeln , dann 

 durch Vereinigung zu einem Sammelgang nach aussen münden. 



Für die Urniere dagegen ist die Bildung besonderer Glomeruli 

 charakteristisch, die in dem Verlauf der Canälchen selbst eingesenkt 

 sind und aus denen die wässrigen Stoffe abgezogen werden." 



In Myœine, therefore, there are strong grounds for believing that 

 the glomerulus and glomus, like the pro- and mesonephric tubules, 

 are dyshomologous structures, since both may coexist side by side in 

 the same segments. When we come to apply Maas' conclusions to 

 Petromyson^ we find that there is no great difficulty. The pronephric 

 glomus, although a multisegmental structure and altogether indepen- 

 dent of the tubules is probably traceable phylogenetically to the same 

 structure as the pronephric capillary nets of Myxine and the lacunar 

 nets of Amphioxus. At present the glomus of Petromyzon would seem 

 to be concerned mainly in secreting the pericardial fluid while the 

 vascular relations of the pronephric tubules are with the anterior 

 cardinal vein in which they are embedded. Perhaps the taking on of 

 this vascular relationship has led to the emancipation of the glomus. 

 In the case of the mesonephros the conditions are essentially the same 

 as those of Myxine: the tubules are supplied with glomeruli by ar- 

 terioles from the aorta and the blood leaving the vascular tufts is 

 spread out in an extensive net-work of capillaries investing the regional 

 homologue of the pronephric tubules, the pronephric duct. The very 

 similar condition of the vascular net-work in Myxine was long ago 

 elucidated by Wilhelm Müller (1875). The young Myxine differs 

 from the young Petromyzon^ or Arnmocœtes, in having the pronephros 

 separated from the pronephric duct of the mesonephric region, some 

 of the pronephric tubules being aborted in the intermediate space. 



1) Spacing in the original. 



5* 



