LAW OF DEVELOPMENT KNOWN AS VON BAEr's LAW. 91 



most important subject, and I propose in a future paper to 

 collect and examine as many cases as I can find of the reten- 

 tion in the embryo of organs which have lately disappeared in 

 the adult. 



There is another aspect of the same question which is sug- 

 gested by the above considerations, viz. if an organ can disap- 

 pear unevenly there is no reason in the nature of things, so far 

 as I can see, why it should not disappear in its developing em- 

 bryonic stages before it does so in the adult, so that there would 

 still be found in the adult a persistent useless rudiment of it 

 after all trace had gone in the embryo. And we may even go 

 further than this, and maintain that if organs can disappear 

 unevenly it is conceivable that traces of an ancient organ might 

 appear and disappear more than once in the course of develop- 

 ment. Of the last-suggested phenomenon I know of more than 

 one instance, but I know of no instance of an organ disappear- 

 ing in its embryonic stages while still persisting as a rudiment 

 in the adult. As an example of the repeated appearance and 

 disappearance of a rudimentary organ in embryonic develop- 

 ment I may mention the neurenteric canal of certain species of 

 birds as described by Gasser/ and quoted by Balfour in the 

 ' Comparative Embryology' (vol. ii, p. 162, mem. ed.). The an- 

 terior neuropore of Ascidians, which appears twice in the 

 development, is another example of the same phenomenon. 

 Although I know of no instance of an organ disappearing in 

 the embryo before it totally disappears in the adult, I do know 

 of instances of rudimentary embryonic organs which have 

 disappeared in their earlier stages while still present at a 

 later stage, e. g. the muscle-plate coelom of Aves, the primitive 

 streak of Amniote blastoderms, and the neurenteric canal of 

 Aves ; and I have no doubt that many instances of this might 

 be collected. 



From the application of the principles set forth in the pre- 

 ceding pages it becomes apparent to us why it is that in the 



' Gasser, " Der Primitivstreifen bei Vogelembryonen," ' Scliriften d. 

 Gesell. zur Beford. d, gesammten naturwiss.,' zu Marburg, vol. ii, sup. 1, 

 1879. 



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