148 Gastrulation and Embryo Formation in Amia Calva 
the anterior end of the fore brain, the continued invagination of the 
superficial epiblast gives rise to a deep pocket in which a lumen is some- 
times plainly apparent, while at other times its walls are so closely ap- 
posed that no lumen is discernible. The fore gut is here well shown 
with its forward extension into the precerebral region where it ends in a 
dilated cavity. The walls of this cavity, except the ventral, are made up 
of elongated hypoblastic cells. Just beneath this median evagination 
there is a large chamber surrounded by a double wall. The lining wall 
is made up of elongated cells which strongly resemble those lining the 
body cavity. Outside this layer is a second wall made up of large yolk- 
laden entoblastic cells. This chamber represents the beginning of the 
heart. The discussion of its formation, however, may best be deferred 
until we have studied the series of transverse sections of the next stage. 
Embryo One Hundred and Twenty-five Hours After Fertilization. 
Embryo Covers 260°.—The last stage of the embryo included in the pres- 
ent study is represented in Figs. 19 and 20, the anterior portion being 
shown in Fig. 19, while the posterior is represented by Fig. 20. The 
embryo has increased greatly in length and its body is more prominent 
above the surface of the yolk, while the tail is just becoming free from 
the yolk. The increase in the length of the head has caused further 
shifting in the position of the adhesive organs which, instead of having 
their surfaces directed above, have come to occupy such an oblique po- 
sition that their surfaces are almost directed forward. ‘The so-called 
“button” (Reighard) is likewise carried forward and is no longer 
visible when the embryo is viewed from the dorsal surface. Just behind 
the adhesive organs are the two nasal pits which are visible for the first 
time in surface views. In the eyes, the lenses are plainly shown in the 
surface views. The mid. brain has extended backward, while the hind 
brain has pushed forward in such a manner that its anterior portion en- 
velops the posterior portion of the mid brain. In this embryo, the roof 
of the hind brain has been removed and one can plainly see in its floor 
a number of neuromeres. These are variable in the different embryos 
of this age, ranging from six to eight. On either side of the hind brain, 
the auditory vesicle shows as a deep pit. Three visceral arches are now 
well defined, as are also the three visceral clefts which appear as darker 
portions between them. The ccelomic cavity shows as a darker circle 
around the margin of the embryo, although its boundaries are not as 
clearly defined as in some of the earlier stages. (Fig. 17). The proto- 
vertebre have extended on either side until they now reach from the ex- 
treme posterior end nearly up to the auditory pits. The pronephric 
ducts have extended both anteriorly and posteriorly. At their anterior 
ends they curve outward, then inward, in the form of a shepherd’s crook. 
