CELL-DIVISION AND GROWTH 389 



pod Crepidula, an animal which varies greatly in size in the mature 

 condition, the dwarfs having in some cases not more than 2V the vol- 

 ume of the giants. The eggs are, however, of the same size in all, 

 and their number is proportional to the size of the adult. The same 

 is true of the tissue-cells. Measurements of cells from the epidermis, 

 the kidney, the liver, the alimentary epithelium, and other tissues 

 show that they are on the whole as large in the dwarfs as in the 

 giants. The body-size therefore depends on the total number of cells 

 rather than on their size individually considered, and the same appears 

 to be the case in plants.^ 



A result which, broadly speaking, agrees with the foregoing, is 

 given through the interesting experimental studies of Morgan ('95, i, 

 '96), supplemented by those of Driesch ('98), in which the number of 

 cells in normal larvae of echinoderms, ascidians, and Aniphioxus is 

 compared with those in dwarf larvae of the same species developed 

 from egg-fragments (Morgan) and isolated blastomeres (Driesch). 

 Unless otherwise specified, the following data are cited from Driesch. 



The normal blastula of Sphcsrechiuus possesses about 500 cells 

 (Morgan), of which from 75 to 90 invaginate to form the archenteron 

 (Driesch). In half-gastrulas the number varies from 35 to 45, occa- 

 sionally reaching 50. In the same species, the normal number of 

 mesenchyme-cells is 54 to 60, in the half-larvae 25 to 30. In Echinus 

 the corresponding numbers are 30 ± and 13 to 15. In the ascidian 

 larvae — a particularly favourable object — there are 29 to 35 (excep- 

 tionally as high as 40) chorda-cells ; in the half-larvae, 1 3 to 17. While 

 these comparisons are not mathematically precise, owing to the diffi- 

 culty of selecting exactly equivalent stages, they nevertheless show 

 that, on the whole, the size of the organ, as of the entire organism, is 

 directly proportional to the number and not to the size of the cells, 

 just as in the mature individuals of Crepidula. The available data 

 are, however, too scanty to justify any very positive conclusions, and 

 it is probable that further experiment will disclose factors at present 

 unknown. It would be highly interesting to determine whether such 

 dwarf embryos could in the end restore the normal number of cells, 

 and, hence, the normal size of the body. In all the cases thus far 

 determined the dwarf gastrulas give rise to larvae {Plutei, etc.) corre- 

 spondingly dwarfed ; but their later history has not yet been suffi- 

 ciently followed out. 



The gradual diminution of the energy of division during develo]> 

 ment by no means proceeds at a uniform pace in all of the cells, and, 

 during the cleavage, the individual blastomeres are often found to 

 exhibit entirely different rhythms of division, periods of active division 

 being succeeded by long pauses, and sometimes by an entire cessa- 



^ See Amelung ('93) and Strasburger ('93). 



