LAW OF DEVELOPMENT KNOWN AS VON BAER’S LAW. 51 
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 devoloping 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 ofa 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, | 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 
1 Gasser, “Der Primitivstreifen bei Vogelembryonen,” ‘Schriften d. 
Gesell. zur Beford. d. gesammten naturwiss.,’ zu Marburg, vol. ii, sup. 1, 
1879. 
