![]() They are segmented and run the length of the body to form the branchial skeleton and sensory nerves. ![]() The neural crest cells are absent on this model. The inner layer of hypomere mesoderm (splanchnic) forms the outer wall of the gut, the outer layer (somatic) lines the body wall. The muscle mass of the somites, epimere myotome, is purple-grey, next to the notochord. The mesoderm grows inbetween the endoderm and ectoderm, eventually forming mesodermal somites. ![]() Model #22 shows a dorsal, tubular nerve chord. Since changes move from anterior to posterior, the oldest part of the section is anterior. It is like adult connective tissue which makes up the bulk of the body. It is not a primary germ layer but in the embryo is amoeboid and can become any layer. The grey represents mesenchyme, embryonic tissue from mesoderm (red) with unfixed potential to differentiate. Later, organs will hang in the coelomic space. The coelom is pouched off (enterocoely) from the archenteron. Model #17 shows neural plate formation, see figure 3, page 9. The yellow endoderm forms the archenteron or primitive gut. The pink on the model is ectoderm (blue). As the cells roll into the dorsal lip of the blastopore they become mesoderm. In model #12 the gastrula (gaster = gut or stomach), the top inner layer of mesoderm is called the chordamesoderm because it induces the notochord. Review early embryology with the morula (little berry) an almost solid ball, the blastula, a hollow ball and gastrulation. AMPHIOXUS MODELS:Įxamine the models of Amphioxus development series 1-23 on demonstration. The chick, has a large-yolked (macrolecithal) egg. The frog has yolk (mesolecithal), meroblastic cleavage and splits the mesoderm to form the coelom. The urchin and Amphioxus the lancelet are without yoke (microlecithal), having holoblastic cleavage and a pouched-off mesoderm. Four species are compared: Models of the sea urchin show the basic pattern. In this laboratory session we are going to study the basic plan of the vertebrate embryo. Look at the diversity of Vertebrates on display in the lab and fill in the cladogram of characters that separate the groups. They have a brain in a brain case with specialized sense organs, closed circulatory syste, chambered and muscular heart, red blood cells, and glomerular kidneys. The notochord may be replaced partially or entirely by vertebrae which protect the dorsal nerve chord. Subphylum Vertebrata is extremely diverse. This is the one group that retains the Chordate characteristics throughout its life. The larva looks like a minature adult without gonads. Study their anatomy and note their chordate characteristics. This marine animal looks very fish-like with fins, a tail, muscle blocks called myomeres and pharyngeal gill slits for feeding. Subphylum Cephalochordata is represented by the little sand lance, Amphioxis. If you look at the tadpole larval stage instead of the adult, you will see many more characteristics of the Chordates. The pharyngeal gill slits form a large filtering basket and the endostyle (thyroid precurser) produces mucus to trap small particles. This is because the adults have become sessile filter feeders. Look at the adult tunicate and you will see very little in the way of Chordate characteristics. The subphylum Urochordata consists of the tunicates and related animals. What are the characteristics of the Phylum?Ĭhordates are subdivided into three subphyla. Many characteristics of chordates can only be seen in the embryonic stages. What was once thought to be a notochord is no longer considered homologous. The Phylum Echinodermata (starfish, urchins, sea cucumbers, feather stars etc.)is an invertebrate phylum but we can look at their early embryology and consider it the same.Īnother Phylum, the Hemichordata (acorn worms) also have pharyngeal gill slits and a dorsal nerve cord, but the nerve cord is not hollow. We presume that other Phyla with this type of development are closely related. Vertebrates are not the only animals with deuterostome development. What does deuterostome mean and what are the characteristics of this type of development? V ertebrates have deuterostome development. Protochordates, overview of Vertebrate phylogeny, and germ layer origin and formation. ![]()
0 Comments
Leave a Reply. |