of the different species do not differ greatly in size. In various ascidians (Styela, Ciona, Molgula, Phallusia, Ascidia) there is a close correspondence in the character and number of the cleavage cells present at corresponding stages of development, even up to advanced stages. For example in all these genera there are 118 cells present when the cupshaped gastrula is first formed and the prospective fate of each of these cells is indicated herewith: 10 will give rise to endoderm cells, 12 to muscle cells, 16 to mesenchyme cells, 8 to chorda cells, 8 to neural plate cells, 64 to ectodermal epithelium.
At the stage when the gastrula begins to elongate there are 218 cells distributed as follows: 26 endoderm cells, 12 muscle cells, 20 mesenchyme cells, 16 chorda cells, 40 neural plate cells, 104 ectodermal epithelial cells.
Each of these cells is characteristic in position, structure, size and potency, and this is true of all species and genera of simple ascidians hitherto studied with respect to this matter.
In a number of species of small body size Martini[1][2][3] has determined that there is a high degree of constancy in the number of cells in the adult body. In the appendicularian Fritellaria pellucida the number of cells is constant in the following organs: 28 flattened epithelial cells of body, 446 oikoplasts (columnar epithelial cells of body), 10 large gland cells in the tail, 7 flattened epithelial cells of the pharynx, 10 large cells of the endostyle, 24 small cells of the endostyle, 4 branchial gland cells, 7 branchial cells, 6 ciliated funnel cells, 19 epithelial cells in the stomach, 10 epithelial cells in the pyloris, 17 epithelial cells in the small intestine, 12 epithelial cells in the large intestine, 6 or 7 epithelial cells in the rectum, 39 cells in the brain, 25 cells in the chief caudal ganglion, 23 cells in the remaining nerve cord, 8 nuclei in heart and pericardium, 20 muscle cells, 12 large chorda cells, 4 small chorda cells.
In different individuals of this species there is a high degree of constancy in the number of these cells, the only variation being in the occasional presence or absence of a single subdivision of a cell.
Also in the rotifer Hydatina senia he finds that there are all together just 959 cells, or rather nuclei, in the entire body of the adult, and that each organ consists of a perfectly characteristic number of cells. Even in different species of rotifers the number of cells in many homologous organs is the same; thus there are generally 6 cells in the anterior part
