PRINCIPLES OF ANIMAL BIOLOGY 43 



2. During the Second Day After Laying. 



2e. Early gastrula. Previous to this stage the cells have through 

 successive division become small and numerous, the whole mass forming a 

 hollow ball known as the blastula. During the early gastrula stage the 

 cells near the border between the black and white areas begin to be tucked 

 into the hollow interior of the mass, along a crescent-shaped line. This 

 crescentic opening is the blastopore. Note that the cells on one side of it 

 are white, on the other side black. Draw, with the blastopore in the 

 middle of the figure, convex side up. Label animal and vegetative poles. 



2/. Late gastrula. The invagination of the cells into the interior is 

 now occurring along a circular line, that is, the blastopore is now a circle. 

 The white cells within this circle constitute the yolk plug. The yolk 

 plug is all that is left of the vegetative half of the egg that has not retreated 

 into the interior. A neural groove may be present, but will probably not 

 be found at this stage. Draw, turning the blastopore nearly to the right 

 side. Label animal and vegetative poles. 



2g. Neural groove stage. The neural fold is a ridge or elevation on the 

 surface of the embryo. The fold is continuous and in the form of an 

 elongated ring, wide at one end and narrow at the other. Later the wide 

 part forms the brain and the narrow part the spinal cord. The groove 

 between the neural folds is the neural groove. At a somewhat later stage 

 the neural folds of the two sides come together above the neural groove 

 and fuse, forming a neural tube, which differentiates into the brain and 

 spinal cord. Draw, with the dorsal side toward you, that is showing the 

 whole nervous system. 



3. About the Fifth Day After Laying. 



3h. Early larva. Note the prominent tail; the V-shaped sucker under 

 the head;&nd the rounded body, its form due to the yolk still present. At 

 each side of the neck there may be a prominence, the gill plate. Draw, 

 in side view, but tilt the ventral side up enough to show the sucker. Omit 

 shading. 



4. About the Eighth Day, or the Time of Hatching from the Jelly. 



4z. Tadpole. Note the external gills developed from the gill plate of 

 an earlier stage; the operculum, a fold of skin partially covering the gills 

 and extending entirely across the ventral side of the body; the broad tail 

 with its thin margin or fin; the V-shaped segments or myotomes into which 

 the muscles in the axis of the tail are divided ; the angular mouth beneath 

 the head; the two suckers formed by the division of the original one sucker; 

 and the eye, a whitish spot surrounded by a darker ring on each side of the 

 head above. The nasal pits, minute depressions at the anterior end of the 

 head, will be visible in clean specimens. 



Draw a tadpole, tilting up the ventral side enough to show the mouth 

 and suckers. Omit shading. 



