156 Gastrulation and Embryo Formation in Amia Calva 
as the anlage of a ligament or a blood vessel. By others it is regarded 
as the remains of a blood vessel or of a blood-forming organ. Still others 
think it entirely disappears. We incline toward the last view, but our 
later stages are not complete enough to settle the question definitely. 
THe HEART. 
In considering the origin of the heart, it is necessary to recall that 
toward the anterior end of the embryo the ecelomic cavities on either side 
approach the middle line. This approximation proceeds anteriorly until 
the two halves of the ccelomic cavities are brought closely together. Just 
before they meet each becomes folded back at the edge. Through this 
folding back of the splanchnopleure there are formed two grooves; the 
edges of these two grooves unite across the median line to form a single 
oval sac which is open both anteriorly and posteriorly. This sac is lined 
by entoderm and surrounded by the splanchnic mesoblast. While these 
changes have been going on there has appeared within the heart cavity 
thus formed a layer of cells which have the appearance of mesoblast 
cells, but apparently they are derived from the hypoblast. Whether they 
are to be considered as mesoblast, that at this relatively late period has 
differentiated from the hypoblast, or whether they are to be considered 
as hypoblast we are unable to say. Only by knowing the fate of these 
layers could one hazard an interpretation. 
THE CENTRAL NERVOUS SYSTEM AND SENSE ORGANS. 
The central nervous system, as observed by Dean, 96, is first formed 
as a solid rod or keel from the deeper ectoblast. Soon after the appear- 
ance of the optic vesicles a lumen is formed, but whether through cyto- 
lysis or delamination or both is uncertain. There are indications which 
lead us to regard cytolysis as the most probable. 
Towards the caudal end of the embryo the superficial ectoblast folds 
downward into the neural keel forming the neural trench, which at the 
posterior end passes over into the blastopore. The question whether or 
not this is to be interpreted as a neurenteric canal depends upon the sig- 
nificance of the neural trench. If this trench is to be considered as 
homologous with the extreme lower part of the medullary grooves in 
Amniota, as Kupffer regards it in the trout, we should certainly consider 
its continuation over into the blastopore as a reminiscence of the neuren- 
teric canal. However, both Wilson’s and Kupffer’s views are questioned 
by Minot and others, and since the interpretation rests upon funda- 
