The development of the urinogenital organs of the lamprey. ß9 



tubules which Felix designates as such would be precociously deve- 

 loped mesonephric tubules provided with true glomeruli. We could 

 perhaps go even a step further and doubt whether what Semon calls 

 the pronephros of Ichthyophis is really such. Certainly he did not 

 study its origin for he did not have the requisite stages, and until 

 these have been studied such an extensive pronephros in an Amphibian 

 may well excite our suspicions. 



Concerning the more fundamental question of the origin in the 

 embryo of the blood-vessels and blood-corpuscles, a subject which 

 naturally presents itself to the investigator who concentrates his at- 

 tention on the center of the Vertebrate body while studying sections 

 of the urinogenital organs, I may be permitted to give my own con- 

 clusions briefly, without attempting to discuss the vast literature of 

 this vexed subject. The blood-vessels arise in the embryo Petromyzon 

 in the entoderm and mesoderm as spaces without endothelial walls. 

 The plasma, which alone first circulates in these spaces, probably 

 comes from the yolk. The blood corpuscles certainly arise from the 

 entoderm and migrate out into the vessels, which at this stage are like 

 primitive lymph-vessels in that they have no endothelium. This as well 

 as the endocardium are formed by certain of the migrating and amoeboid 

 blood-corpuscles settling down on the walls of the cavities, flattening 

 themselves out and becoming fitted together at their edges. The 

 conditions in Fetromyzon, except that the blood-corpuscles do not, in 

 the first instance at least, arise from the mesoderm, suggest those 

 observed by Wenckebach (1886) in the living Teleost {Belone) 

 embryo: "Man sieht deutlich, wie die Zellen, namentlich des Meso- 

 blasts, selbständig mittelst amöboider Bewegungen und oft ausser- 

 ordentlich langen protoplasmatischen Fortsätzen sich im Körper des 

 Embryo und auf dem Dotter bewegen und nach bestimmten Stellen 

 kriechen, als handelten sie mit Willen und Bewusstsein. Bei der An- 

 lage der Endothelzellen des Herzens und der grossen Gefässe des 

 i?eZowe-Embryo spielen diese wandernden Zellen eine grosse Rolle." 

 In regard to the question as to whether the blood-vessels originate 

 in situ or by immigration, I agree with Felix only so far as the 

 mesodermal walls are concerned, when he writes (1897 b, p. 365): 

 'Teh selbst kann zufügen, dass der Glomerulus der Hühnervorniere 

 zunächst unabhängig von der Aorta in loco entsteht. Ich kann weiter 

 behaupten, dass Stammvene, Venenplexus der Vorniere, das Eigengefäss 

 des Glomerulus und damit die A. mesenterica, die Aorta bei Salmo- 

 niden in loco entstehen. Ueber die Anlage des Herzens und der 



