154 Gastrulation and Kmbryo Formation in Amia Calva 
Dean, 96, who says: “ The mode of origin of the sucking dise gives the 
most interesting evidence of how precociously embryonic and_ larval 
structures may be developed. As far as histological evidence goes there 
is certainly no difference between the enlarged thick-walled, cup-shaped 
organs which arise on the snout of the late embryo of Amia or of Lepi- 
dosteus, and the typical pit organs, or sense buds, which later occur on 
other integumental regions. It is found, in fact, that a gradation in 
size exists which connects the huge sucking organs of the snout with the 
inconspicuous pit organs of the trunk.” 
These organs were later studied in Reighard’s laboratory by Miss 
Phelps, 99, who found that “the organ is formed in a very early stage 
as a diverticulum of the fore gut. This diverticulum subsequently di- 
vides into two, each of which continues to communicate for a time with 
the cavity of the fore gut.” The author further observed that the organs 
open to the exterior, but become cut off from the fore gut and degenerate 
leaving no trace behind. 
Our studies show that these organs arise from paired diverticula of 
the fore gut and not from a single diverticulum which later divides. 
While it is undoubtedly correct to state that the paired gut diverticula 
are derived from an unpaired condition, there is not the slightest evi- 
dence that the anlagen of these structures appear before the gut diver- 
ticula are well established. The beginnings are first visible as slight 
thickenings of the hypoblast, forming the antero-dorsal walls of these 
diverticula. As development progresses these thickened areas evaginate 
and the cells begin to elongate. Soon a longitudinal constriction forms 
which divides each of these structures, giving rise to four. Meantime 
the lumen of each is reduced, the walls of the gut become apposed and 
the organs are cut off from further communication with the gut. After 
losing their connection with the gut they continue to divide until eight 
or more are formed. They then come in contact with the ectoblast whose 
cells undergo cytolysis, leaving the hypoblastic cells of the organs pro- 
jecting to the free surface. We have not followed the later changes in 
these organs. Miss Phelps states that after being functional for a time 
the organs are pushed beneath the surface of the thickened ectoblast, be- 
come filtrated with leucocytes and finally disappear. 
As to the meaning of these remarkable organs we are in the dark. 
Their function may be only to hold the larva in position for a certain 
period. Again they may serve to convey some sort of nutriment to the 
digestive tract. That they are modified sense buds, as Dean suggests, 
seems highly improbable. Their interpretation from a phylogenetic 
standpoint is certainly most difficult. About all that can be said is that 
