84 MORPHOGENESIS IN CILIATES 



We know that in euglenas such as Euglena jnesnili, the 

 relative speed of chloroplast multiplication and of cell divi- 

 sion may be altered in the absence of light [A. Lwoff and 

 H. Dusi (1935)]. This is an example, or a model, allowing 

 us to accept for the time being the hypothesis that, owing 

 to the nature of reserves, that is to say, of metabolism, cyto- 

 plasmic corpuscles can or cannot multiply, or multiply more 

 or less rapidly, and in some instance be lost in certain types 

 of cells. During development of the egg of the ctenophore 

 Beroe or of the mollusc DentaUum, substances such as 

 ^'green ectoderm-producing substances" or '^non-colored en- 

 doderm-producing material" are synthesized. Whether 

 they are located in, or bound to, plasmagenes is not known. 

 Nevertheless, as these substances are unequally distributed, 

 cleavage brings about a segregation or localization of pre- 

 formed substances. During the development of the trocho- 

 phore larvae of molluscs, only some lines of cells produce 

 cilia. Obviously, kinetosomes have multiplied and have 

 produced cilia in certain types of cells only. Thus an or- 

 ganelle, the kinetosome, endowed wdth genetic continuity 

 may behave differently in different cells of an animal, just 

 as it behaves differently in different parts of a ciliate. Are 

 these two types of "differentiation," to some extent, com- 

 parable? Differentiation in a ciliate is essentially a cortical 

 phenomenon. To what extent is it possible to extend data 

 concerning ciliates to other animals? Is the cortex of a 

 ciliate something peculiar? Are the cortical phenomena 

 which occur in ciliates the specific expression of a general 

 law? 



The importance of cell surface has been stressed by many 

 embryologists. E. E. Just (1939) went so far as to consider 

 that "in the entire animal kingdom, with the exception of 

 mammals, the embryo arises from the egg surface." E. 

 Faure-Fremiet and H. Mugard (1948) have discovered that 

 very important cortical phenomena take place during the 



