Development of a Salamander, Amblystoma punctatum
(Later called Ambystoma maculatum)
Watch the Development of a Salamander 8.5 minutes.
This first movie was produced by Leon Stone and T.C. Kramer in 1924 at Yale University Anatomy Department. It was on 16mm movie film. At that time, Stone was an Instructor and later a Professor at Yale. He was one of the distinguished students of Ross Harrison at Yale. Harrison was the leading embryologist in America at the time.
I bought a copy of the 16mm film from Dr. Stone when he was still alive in the 1960's. I showed the movie to classes at the University of Texas for 40 years. After I retired we had the movie digitized and burned it onto a DVD. The movie begins with a view of ponds near Yale where the salamanders breed.
Development begins with a zygote (a single-celled fertilized egg) 2mm in diameter. The zygote divides into ever-smaller daughter cells in a process called cleavage. Yolk stored within each cell provides nourishment and substances used to form new cells. The entire embryo does not grow until it hatches and feeds.
Cleavage continues into blastula stages, which are characterized as being a ball of cells enclosing a cavity, the blastocoel, in the upper (animal) hemisphere. Cells near and beneath the equator then move toward the forming blastopore and turn under the dorsal lip of the blastopore. The endoderm simultaneously rotates into the interior beneath the dorsal lip of the blastopore. The blastopore lip initially forms dorsally, then extends laterally, and finally forms ventrally to complete a round blastopore filled by a yolk plug. The blastopore is in the position that is later the site of the cloacal opening.
The process of moving the equatorial mesoderm and the ventral endoderm within the embryo is called gastrulation. The neural ectoderm remains on the surface dorsally, and the epidermal ectoderm occupies the surface of the ventral hemisphere.
The neural ectoderm is one cell thick and initially covers the dorsal hemisphere. As the neural plate forms, these cells double their heights which reduces the surface they cover by half, thus converting the dorsal hemisphere into a thicker flat disc of the same diameter. This is the beginning of neurulation.
Over the next 40 hours, the neural disc reshapes itself into a pear-shaped flat neural plate. In the next five hours, the neural plate elongates and rolls up into a hollow neural tube, completing neurulation.
The embryo lengthens into a fish-like shape, a larva. Organs form and limbs, fins, gills, balancers, etc. protrude outward. The first organ to function is the beating heart at stage 34. The larva hatches from its protecting vitelline membrane and surrounding jelly at stage 40, competing the first four weeks of development. The hatchling then feeds at first opportunity.
Antone G. Jacobson
Research in our laboratory found that neurulation occurs in two phases, namely, the conversion from a flat circular neural plate at stage 13 to a flat pear-shaped plate at stage 15, then the rolling up of the flat plate into a neural tube between stages 15 and 20. The first phase, between stages 13 and 15, takes 40 hours, and the second phase, between stages 15 and 20, takes five hours in the West Coast newt, Taricha torosa at 17 degrees Celsius. We have pieced together excerpts from several movies taken of these two phases of neurulation in the following video.
Watch the movie Neurulation Occurs in Two Phases 13.5 minutes.
The fertilized egg of the West Coast newt, Taricha torosa, is about 2 mm in diameter. During the process called cleavage or segmentation, the egg divides in a series of simultaneous divisions, first into two cells, then into four, then eight, etc. Some examples of cleavage are shown in this next video.
Cleavage six minutes.
This movie shows embryos answering the question "If the embryo is cut in two across the long axis, will each half continue to neurulate?"
This unedited movie starts with an intact newt embryo at stage 15 just before it was cut in two. The embryo is then shown cut in two and followed as the parts neurulate.
The next scene shows an earlier stage, stage 14, just after being bisected. The massive endoderm is distracting as it heals itself, but the neural plates on the two parts continue neurulation.
The final scene shows a stage 15 embryo cut across the long axis through the neural plate, the underlying mesoderm, and the top of the mass of gut endoderm, leaving most of the embryo intact.
Watch the movie Neurulation in Half Embryos 2.5 minutes.