Development of Planocera

Planocera belongs to the phylum Platyhelminthes, class Turbellaria and order Polycladida. The development of this organism is completely studied. Its zygote will undergo spiral clevage.

1. Clevage is complete. The first two divisions are meridional and are at right angles to each other. Four cells are formed, out of the four cells, the posterior cell is biggest and the remaining three are small.
2. To notice their fate these are named with alphabets.
3. These four Cells are called with A, B, C, and D names.
4. "D" cell is the biggest.
5. Third cleavage is horizontal. 8 cells are formed. Out of 8 cells, 4 are micromeres and 4 cells are macromeres. 1a, 1b, 1c, and 1d will form the 1st quartet.

   

6. This 8-celled stage will undergo another cleavage and 16 celled stage is resulted.
7. This 16-celled stage will undergo another division and 32 celled stage is formed.
8. At this stage 3A, 3B, 3C, 3D Macromeres and 1, 2, and 3 quartets of micromeres are present. The cells of the first three quarters will give the epidermis. This embryonic stage is called Gastrula.

   

9. Again a cleavage is carried on 4a, 4b, 4c and 4d cells are formed. This is called the 4th quartet.
10. 4A, 4B, 4C, 4D, 4a, 4b, and 4c cells will disintegrate and form food for the developing embryo.

    

11. The 4d cell is big. It is called a Mesoblast cell. It will divide and give 4d, and 4d² cells. 4d> will divide and give 4d" 4d¹² will give endoderm of the intestine.

    

12. 4d²¹, 4d²² cells will give endoderm of the pharynx and also mesoderm. This mesoderm will give muscles, mesenchyme, and reproductive systems.

    

The fate of different cells - 1

Then the gastrula transforms into Müller's larva.

Muller's larva:

1. This larva is oval. It freely swims in the seawater.
2. On its ventral side mouth is present
3. On this larva ciliated band is present. This ciliated band is useful for swimming.
4. At the apex a bunch of cilia will be present. This bunch is called an apical tuft.
5. Below the apical tuft of cilia two eye spots are present.

This larva will swim for 4 or 5 days in the water. Then it will transform into an adult.