After certain number of divisions the macromeres give rise to a pair of blastomeres termed as teloblasts M, N, Q and O/P. It is from these teloblasts that the segments of the annelid are derived in the adult.
To draw our fate map we used alkaline phosphatase to help in tracing out the cell lineage of many of the adult structures. Alkaline phosphatase being naturally present in a cell and was an ideal candidate as a marker to trace out the fate map. The blastomeres were injected with high concentrations of alkaline phosphatase and then allowed to develop into their corresponding adult structures. The annelid was then bathed with a colourless substrate which coloured on reacting with alkaline phosphatase.
Using this we were able to show that the blastomere A, B & C most probably develops into the ectoderm. The blastomere D though proved to be the most vital with most of the endoderm and mesoderm structures being derived from it. We also show the individual fates of each of the teloblasts and by alternating the time of injecting alkaline phosphatase we were able to deduce many vital facts about metamerisation. From our study we came to the conclusion that most probably the segments are produced from the posterior to the anterior direction and that segmentation begins at a very early stage of asymmetric cell division of the teloblasts.
The phylum Annelid consists of many species that range from the humble earthworm all the way to the maligned leech, but one common feature that binds them all is their segmented body. This segmentation is termed as Metamerism and each ring like segment is called a metamere. In fact the word Annelid is derived from the Latin word Annelis meaning rings. The Annelid body can be further bisected into two equal halves implying that the body architecture is bilaterally symmetrical. Annelids are also triplobalstic i.e. the embryo has 3 distinct germ layers namely the mesoderm, endoderm and ectoderm. It is from these 3 distinct germ layers that all the organ systems arise. The ectoderm that forms the outer layer of the embryo produces the epidermis and the nervous system, the endoderm which forms the inner most layer of the embryo gives rise to the digestive system and its related organs and sandwiched in between the two layers is the mesoderm which produces the circulatory system, muscles and connective tissue.
As the yolk in the Annelid embryo is equally spaced out a Holoblastic cleavage pattern is seen and thus the cleavage furrow extends through the entire egg. The cleavage pattern seen in Annelids is a Spiral Cleavage which is also seen in other organisms such as sea urchins. During the first of cleavage, the embryo is divided into 4 cells each of which is called a blastomere and are labelled as A, B, C and D. All the blastomeres are of equal size except for Blastomere D which is slightly larger than the rest. The second round of cleavage produces an unequal division of 4 large blastomeres called Macromeres and 4 smaller blastomeres called Micromeres. The macromeres are labelled as A1, B1, C1, and D1 and the micromeres are labelled as a, b, c, and d. The macromeres keep on dividing to produce more macromeres and micromeres and eventually the entire organism is formed.
Of special note is the D1 blastomere which further divides to