In our series this week on note taking, we have moved to day two. Todays lecture discussed differences in creatures in the deuterostoma category – particularly what happens after the notochord forms.
This is AV’s summation:
Bilateral symmetry is in all living things with a notochord – fish, lizards, people, etc. Echinoderms and a few other animals are radially symmetrical. Asymmetrical is when there is not symmetry like most sponges, some larvae and most alga.
To be a chordate an animal must have four characteristics: 1. Notochord – the notochord is often lost in the adult; 2. nerve chord – the nerve chord is not lost; 3. gill slits – in land animals it is lost as an embryo; and 4. postanal tail (debatable).
The embryo has three parts to its blastosphere: the gut (surrounded by the endoderm), the middle (mesoderm) and the outer wall (ectoderm). In the endoderm, a bulge begins to form, and eventually separates and this chord becomes the notochord. The notochord is now floating in the middle of the mesoderm. It influences the ectoderm to being to form a dip which grows to be a valley and finally inverts and forms a round doughnut. All this forms in the mesoderm too. The notochords job is done, and it dissolves. The doughnut becomes the nerve chord.