The two pictures to the left are basically the same image, however, one is taken using bright field microscopy (top) and the other - using dark field technique (bottom). At this time the larva was just over 7 weeks old. To orient you, this is a ventral view with the anterior to the right, posterior to the left, right side at the top, and the left side on the bottom. The large non-transparent circle in the center is the stomach and right below the stomach is the juvenile rudiment. This is the developmental milestone I am referring to above. The juvenile rudiment usually develops on the left side, except in rare cases, and will go on to become the juvenile urchin. It will develop tube feet and spines and eventually protrude from the larval body. The juvenile will reabsorb most of the larval body in a process known as metamorphosis, and will walk way on its tiny tube feet. At the most posterior end of the larva there is a thick ciliated band called the epaulette. It assists the larva in locomotion, and it if you look closely at both ends you can see the cilia.
This picture is of a different experimental urchin. This is a dorsal view which is why the juvenile rudiment is above the stomach rather than below it. Also, this picture was taken with a 4x magnification lens while the previous were taken with a 10x. (The larvae were approximately the same size). The larval arms aren't quite as straight as on the previous pictures - maybe it was waving to the camera. However, what is interesting about this larva is the size of the juvenile rudiment. It's about the same size, if not larger, than the stomach. It is not uncommon for larvae of the same age to have rudiments of different sizes. Despite the fact that our Embryology class is ending on June 7th, I will continue to rear these larvae. I hope my next pictures are an illustration of urchin metamorphosis!
See also my earlier posts on blastomere separations: Part 1 Part 2