128 



GREGORY. 



[VOL. I. 



adi-lm 



ad.2- 

 adx- 



M:i ' 



& 



, 



a 



granules resembling yolk, mentioned 

 by Balfour and other observers, are 

 very conspicuous. They occur in 

 the somatopleure as well as in the 

 splanchnopleure. 



From the facts here given it seems 

 reasonable to conclude that the earli- 

 est Anlage of the pronephros fuses 

 temporarily with the ectoderm, and 

 may possibly receive some few cells 

 from it. The first part of the duct 

 proper, lying just beyond the last 

 pronephric tubule, seems necessarily 

 to share to some slight degree in the 

 mesodermal origin of the anterior 

 region. This follows from the fact 

 that there is no line of division be- 

 tween the two regions of the duct, 

 and that Rabl and other observers 

 have shown that karyokinesis occurs 

 throughout the length of the duct. 

 Most important of all, we must con- 

 clude that so far as the duct develops 

 distally, the connection of its tip 

 with the ectoderm is maintained. 

 The mere fact of the actual fusion 

 of the tip of the duct with the ecto- 

 derm throughout its growth would 

 be sufficient evidence of a genetic 

 relation for those who accept the 

 principle of the teloblastic growth of 

 organs. The position of the prone- 

 phros in AcantJiias seems to be 

 identical with that in Pristinrus as 

 found by van Wyhe and Rabl. It 

 will be seen that the results here 



FIG. 8. a = aorta, ad = aortic diverticulum, ft = 

 pronephric tubule, -S = somite, x = fusion of duct 

 with ectoderm. 



