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Development of Mouth and Gills in Bdellostoma 
by some organ that he may have studied. Enough. however, has been 
said to show the need of further embryological observations. 
METHOD AND MATERIAL. 
The material upon which the present results are based was collected in 
the Bay of Monterey, California, mainly during the summers of 1896 
and 1899 by Professor Bashford Dean. This constitutes the most 
complete series of Bdellostoma embryos that has been procured. The 
younger stages were fixed for the most part in Graf’s picro-formalin, the 
1ater embryos being killed, after removal from the parchment-like egg- 
shell, in sublimate-acetic. In most cases these fixatives have given very 
satisfactory results. 
The sections were prepared by Prof. Dean, Dr. L. Neumeyer, of 
Munich, and Dr. N. Yatsu, formerly of this laboratory. The embryos 
are very difficult to prepare, but with such an abundance of sectioned 
material I have been able to select a complete and satisfactory series, 
and I am confident that none of my observations are open to the criticism 
that they have been made upon defective material. The stains used were 
Heidenhain’s iron hematoxylin, Delafield’s hematoxylin, alum and borax 
carmine, al! of which give good results. 
THE DEVELOPMENT OF THE MANDIBULAR ARCH. 
In the young of Bdellostoma one finds, to my mind, a most striking 
confirmation of Dohrn’s view that the mouth of vertebrates is a modi- 
fied gill arch. In these embryos the mandibular cleft passes through a 
series of changes during its development which are most suggestive in 
the light of Dohrn’s thesis. Very young embryos, such, for ex- 
ample, in which the nose is still a single tube leading directly into the 
mouth cavity and in which six or seven gill slits are present on the later- 
ally outspread plates (as illustrated in Dean’s Fig. 37, pl. 18, and Fig, 
101, pl. 22), will show the mandibular cleft in the following condition : 
Throughout the greater portion of its extent it is strongly arched with its 
lateral diverticula directed in an oblique dorsal direction and coming in 
intimate contact or actually fusing with the ectoderm. This is so 
strikingly analogous to the way in which a gill cleft meets the ectoderm 
that no observer could fail to be impressed with the similarity. Fig. 1, 
a cross section through the widest portion of the mandibular cleft of an 
embryo at this stage, shows the cleft fusing laterally with the ectoderm 
and having an arch or curve resembling that of all the following gills. 
The hyomandibular of the same embryo may be seen for comparison in 
