512 Development of Mouth and Gills in Bdellostoma 
to. ‘The club muscle develops entirely between the dotted lines and the 
first gill, being thus confined to this area of rapid growth. 
As stated in a previous paper on the development of the thyroid gland 
in Bdellostoma, this gland and the other organs of the gill region develop 
entirely im situ not shifting or moving during their development. For 
the gill shifting process is not an actual moving of the gills as will 
be clearly understood from the explanation which has just been given. 
CONCLUSIONS. 
My studies up to this point have not yet made it possible for me to 
satisfactorily interpret Kupffer’s, oo, “ primary mouth” in Bdellostoma, 
a structure he describes from very young embryos. But I must admit 
that I am extremely skeptical regarding this earliest mouth. At times 
I am led to believe that this was really the nose which in the case of 
very early embryos, as I find, leads directly into the throat. The most 
convincing thing in Kupffer’s argument is the following paragraph re- 
garding the “secondaire Rachenhaut,” a structure which undoubtedly. 
exists. 
“ Bei dieser Sachlage warfen sich verschiedene Fragen auf. Zunichst 
die, ob die Kpithelplatten, durch welche beide Canile ausserlich ge- 
schlossen werden, primaire Bildungen sind, oder ob es sich hier um einen 
secundaren Verschluss vorher klaffender Oeffnungen handelt. Im 
ersteren Falle ware die den Munddarm verschliessende Platte als Rachen- 
haut aufzufassen und am Hypophysencanal fiinde sich eine analoge 
Vorrichtung. Dann aber ware das Epithel beider Canale ein endoder- 
males und es ergabe sich hieraus der paradoxe Schluss, dass das Epithel 
der Nase nur an das Emdoderm Anschluss hatte. Viel einfacher wiirde 
sich aber die Deutung vorhegender Verhialtnisse gestalten bei der An- 
nahme, dass beide Canale erst secundar sich geschlossen haben.” 
Dean, 99, also states on page 270, referring to an old embryo that, 
“Tt seems clear to me, however, that the mouth and nasal openings in 
the present instance are certainly to be looked upon as of secondary 
acquisition.” I must express my conviction as to the truth of the two 
above statements, and must regret that I have not been able in the young 
embryos to find the satisfactory stages showing the primary mouth open- 
ing and its subsequent closure by the ‘* secundiire Rachenhaut.” 
There is much in these embryos to suggest the correctness of Dohrn’s 
idea that the nasal tube, naso-hypophyseal canal, in myxinoids is the 
homologue of the ancient vertebrate mouth. 
Regarding the relationships of the myxinoids I should say that it is 
