Protochordates, overview of Vertebrate phylogeny, and germ layer origin and formation.

Return to Index PROTOCHORDATES: Vertebrates have deuterostome development. What does deuterostome mean and what are the characteristics of this type of development? Vertebrates are not the only animals with deuterostome development. We presume that other Phyla with this type of development are closely related. 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. Another Phylum, the Hemichordata (acorn worms) also have pharyngeal gill slits and a dorsal nerve cord, but the nerve cord is not hollow. What was once thought to be a notochord is no longer considered homologous. VERTEBRATES: Vertebrates are in the Phylum Chordata. Many characteristics of chordates can only be seen in the embryonic stages. What are the characteristics of the Phylum? Chordates are subdivided into three subphyla. The subphylum Urochordata consists of the tunicates and related animals. Look at the adult tunicate and you will see very little in the way of Chordate characteristics. This is because the adults have become sessile filter feeders. The pharyngeal gill slits form a large filtering basket and the endostyle (thyroid precurser) produces mucus to trap small particles. If you look at the tadpole larval stage instead of the adult, you will see many more characteristics of the Chordates. Subphylum Cephalochordata is represented by the little sand lance, Amphioxis. This marine animal looks very fish-like with fins, a tail, muscle blocks called myomeres and pharyngeal gill slits for feeding. Study their anatomy and note their chordate characteristics. The larva looks like a minature adult without gonads. This is the one group that retains the Chordate characteristics throughout its life. Subphylum Vertebrata is extremely diverse. The notochord may be replaced partially or entirely by vertebrae which protect the dorsal nerve chord. 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. Look at the diversity of Vertebrates on display in the lab and fill in the cladogram of characters that separate the groups.

EMBRYOLOGY: In this laboratory session we are going to study the basic plan of the vertebrate embryo. Four species are compared: Models of the sea urchin show the basic pattern. The urchin and Amphioxus the lancelet are without yoke (microlecithal), having holoblastic cleavage and a pouched-off mesoderm. The frog has yolk (mesolecithal), meroblastic cleavage and splits the mesoderm to form the coelom. The chick, has a large-yolked (macrolecithal) egg.   AMPHIOXUS MODELS: Examine the models of Amphioxus development series 1-23 on demonstration. Review early embryology with the morula (little berry) an almost solid ball, the blastula, a hollow ball and gastrulation. In model #12 the gastrula (gaster = gut or stomach), the top inner layer of mesoderm is called the chordamesoderm because it induces the notochord. As the cells roll into the dorsal lip of the blastopore they become mesoderm. The pink on the model is ectoderm (blue). The yellow endoderm forms the archenteron or primitive gut. Model #17 shows neural plate formation, see figure 3, page 9. The black mass is the notochord (red). The coelom is pouched off (enterocoely) from the archenteron. Later, organs will hang in the coelomic space. The grey represents mesenchyme, embryonic tissue from mesoderm (red) with unfixed potential to differentiate. It is not a primary germ layer but in the embryo is amoeboid and can become any layer. It is like adult connective tissue which makes up the bulk of the body. Since changes move from anterior to posterior, the oldest part of the section is anterior. Model #22 shows a dorsal, tubular nerve chord. The mesoderm grows inbetween the endoderm and ectoderm, eventually forming mesodermal somites. The muscle mass of the somites, epimere myotome, is purple-grey, next to the notochord. The inner layer of hypomere mesoderm (splanchnic) forms the outer wall of the gut, the outer layer (somatic) lines the body wall. The neural crest cells are absent on this model. They are segmented and run the length of the body to form the branchial skeleton and sensory nerves. The right side of the model shows a slightly more developed view with a pinched-off mesodermal space in the somite.

FROG MODELS: Examine the models of Frog development (series 1-15), noticing the influence of yolk on cleavage. The neural crest cells are shown in these models. The notochord (red) is pink and the ectoderm (blue) is brown. Note the gut buried in the yolk. The mesoderm grows as a sheet, the coelomic cavity appearing later by schizocoely and segments only appear in the dorsal region.   CHICK EMBRYO SLIDES There are demonstration slides of chick whole mounts and cross-sections on the side bench. A chick has discoidal cleavage in which the yolk does not divide. In the whole mount, the somites are repeated blocks of muscles called myotomes. In this view you see the structure of the epimere mesoderm. Draw and label below the three parts of epimere mesoderm in cross section and what they are most likely to become.   Draw and label below the three parts of the mesoderm in a cross-section of a typical animal, including the somatic and splanchnic layer and indicate what they are most likely to become. The chick cross section is somewhat like the adult germ layer structures in your lab book, but the drawings posted in the lab are better. The chick is sitting on top of the yolk and the gut is open to the yolk. Note the slit-like coelom. The mesoderm is composed of the three parts drawn above.   LAMPREYS: Lampreys are primitive cartilaginous fish, without jaws, called agnatha (a=without, gnatha=jaws). They have soft, scaleless skin commercially called eelskin. Inside they have a cartilagenous skeleton and a muscular body divided into myotomes. They are fish parasites and rasp with a horny mouth to suck blood. Since blood is easy to digest, they are all gut, no stomach, which exits in a common cloaca with the kidney and gonads. There are whole mounts of lampreys and cross sections. There are a series of lamprey cross sections cut posterior to the heart in plastic mounts on the bench. View using a dissecting microscope. The cross sections are from the trunk region. The liver is anterior so it is probably not in your sections. They are dioecious (separate sexes). The gonads, in all sections, are small in the males (testes), large and pebbly in the females (ovaries). The gut is a tiny hollow tube with endoderm inside and mesoderm outside. The kidneys are paired and large, blue on the top because of the injected blood supply. The larval lamprey always lives in fresh water where it burrows and filter feeds. It is called an ammocoetes larva and bears a strong resembrance to the sand lanclet. Examine the specimens on slides in the lab. Unlike Amphioxis, the pharynx has muscles and cartilagenous bars so it is capable of pumping action. Other improvements are the presence of a liver, gall bladder and heart as well as pronephric tubules.The brain, even in the larva is composed of three parts (forebrain, midbrain and hindbrain). After 3-7 years the larva metamorphoses into a juvenile and migrates to the sea or in freshwater species to lakes to take up its parasitic existance. In the following labs, it will be important to know which adult structures are formed from the three embryonic germ layers. The ectoderm is traditionally coloured blue in embryos, the mesoderm is red and the endoderm is yellow. It may help if you colour your lab handouts in the traditional colours. Study the chart of the embryonic origins of adult structures. We will be looking at these structures in detail in future labs, but you should always be aware of their germ layer origin.       Updated by Sandra Millen, August/03 Return to Index