1 84 THE POPULAR SCIENCE MONTHLY 



of the different species do not differ greatly in size. In various ascid- 

 ians (Styela, Ciona, Molgula, Phallusia, Ascidia) there is a close cor- 

 respondence 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 cup- 

 shaped 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 318 cells 

 distributed as follows: 26 endoderm cells, 12 muscle cells, 20 mesen- 

 chyme 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 6 ' 7 ' 8 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 con- 

 stancy in the number of these cells, the only variation being in the occa- 

 sional presence or absence of a single subdivision of a cell. 



Also in the rotifer Eydatina serda 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 



'Martini, "Die Konstanz histologische Elemente bei Nematoden, etc.," 

 Verh. Anat. Gesell, 22, 1908. 



7 " Darwinismus und Zellkonstanz, ' ' Sits. u. Abh. naturforsch. Gesell. 

 Bostock, 1, 1909. 



3 ' ' Studien iiber die Konstanz histologischer Elemente, ' ' I., II., III., Zeit. 

 wiss. Zool., 92, 94, 1909 ; 102, 1912. 



