The development of the urinogenital organs of the lamprey. 73 



The conditions which in the ancestral Craniote led to the ab- 

 andonment of the paired nephropores in the ectoderm may, perhaps, 

 be imagined. We may start with a form like Amphioxus, which has 

 a fully developed atrial cavity, arising from an infolding of the ecto- 

 derm and receiving on either side the openings of the nephridia. 

 This is the condition from which Boveri starts in his attempt to 

 derive the pronephric ducts from the walls of the atrial chamber, an 

 explanation which subsequent researches fail to confirm. Given, 

 however, the atrial chamber and the simple conditions of the nephridia 

 and reproductive organs of a form like Amphioxus, we may perhaps 

 suppose that the following changes came about. The atrial chamber 

 may have been used as a brood-pouch for storing or rearing the ova, a 

 condition still found among certain Tunicates (Clavellina, Synascidia). 

 The great number of ova accumulating periodically in the atrium may, 

 for obvious reasons, have rendered that cavity, at least the greater 

 portion of it, less desirable as a place of opening for the excretory 

 organs. Some other means would have to be resorted to, and may 

 have been found in the fusion of the pronephric tubules with one 

 another, and the obliteration of all but a single pair of openings at 

 the posterior end of the atrial cavity. These openings were sub- 

 sequently shifted to the cloaca. Perhaps the Selachians still show 

 the backward movement ot these ectodermal openings in their 

 unique development of an ectodermal duct, but if this be the case, 

 we have in the shark the retention of a very ancient condition long 

 after all traces of the atrial chamber have disappeared from the onto- 

 geny of the Craniote. Not only have the nephropores moved back 

 to open into the cloaca, but there is also a pronounced tendency as 

 we ascend the Vertebrate phylum to shift the position of the pro- 

 raeso- and metanephros further and further back so that their con- 

 duits become relatively shorter and shorter. In Cyclostomes the pro- 

 nephric funnels open into the pericardium, in the Ichthyopsids some- 

 what further back. In sharks, finally, the excretory portion of the 

 mesonephros, which some investigators have homologized with the 

 metanephros of Amniotes, lies in the extreme posterior portion of the 

 body cavity. 



Pari passu with the phylogenetic and ontogenetic caudalization 

 — if I may use such an expression — of the nephric system, the 

 Gnathostome reproductive organs show an increasing tendency to 

 become inplicated with the excretory organs. This association of 



