PLATE 1 
EXPLANATION OF FIGURES 
Figures 1 to 10 were drawn with oil-immersion objective and reduced a little 
over one-half in reproduction. They illustrate various stages in the develop- 
ment of taste-buds, pharyngeal scales, and the formation of mouth and gill-slits. 
1 An early stage in the development of a taste-bud and papilla as seen in a 
section of a60-mm. Squalus embryo. Differentiation begins as a process of elon- 
gation and aggregation of a group of cells in the stratum germinativum of the 
pharyngeal epidermis which is of endodermal origin. 
2 A section of an older taste papilla taken from a 90-mm. Squalus embryo, 
showing a portion of the associated sensory nerve. 
3 Asection of a fully differentiated taste papilla and ‘bud’ as seen in the ‘pup’ 
stage of Squalus. 
4 A section of the pharyngeal lining of a ‘pup’ stage, showing to the left 
the anlage of a pharyngeal scale and to the right a (somewhat damaged) taste 
papilla. 
5 A portion of a median sagittal section of a Squalus embryo (Balfour’s stage 
K), showing the relations between ectoderm and endoderm in the region of the 
degenerating buccal plate (‘preoral lobe’). The presence of large yolk granules 
in the endodermal cells serves at this stage to distinguish them from the ecto- 
dermal cells of the hypophysial invagination (above). 
6,7, and8 Successive stages in the formation of a visceral cleft, showing the 
ectodermal-endodermal relations in frontal sections of a Squalus embryo (stage 
K). It is the endoderm which persists longest in the region of perforation of 
the cleft. At this stage the ectoderm stains more intensely than the endoderm 
so that their limits are easily distinguishable. The endoderm is the more active 
layer of the two in the process of visceral cleft formation. : 
9 A developing scale as seen in a section of an older ‘pup’ stage through the 
pharyngeal epidermis. 
10 Anenlarged portion of the upper jaw of a60-mm. Squalus embryo, showing 
the region of transition between ectoderm:and endoderm. At neither this nor 
any other stage it is possible to discover evidence of degeneration of the endo- 
derm or of the rapid proliferation of ectoderm which would manifest itself if the 
ectodermal ingrowth were to supplant the primary endodermal lining. 
