CELLULAR DIFFERENTIATION AND INTERNAL ENVIRONMENT 



83 



blastomeres there may also be a restricted 

 regulation, as shown by the movement of 

 myotome cells around the tip of the noto- 

 chord to the defective side (Cohen and 

 Berrill 1986). Evidence of induction of 

 cerebral vesicles with sensory cells by 

 means of certain underlying cells has also 

 been obtained (Rose, 1939 ; Vanderbroek, 

 unpublished^), v. Ubisch (1939), how- 

 ever, failed to find such induction in 

 Ascidiella. 



Again, in Clavelina, which probably also 

 develops through a strictly "determinate" 

 cleavage, there are certain totipotent re- 

 serve cells which enter into the winter buds 

 in unorganized clumps and later give rise 

 to new ascidiozooids (Spek 1927; Brien- 

 Gavage 1927). In regeneration also, the 

 several types of cells and tissues show much 

 lability in their powers of differentiation. 

 These facts indicate that the strictly mosaic 

 cleavage pattern is only of temporary sig- 

 nificance as far as germinal localization is 

 concerned. There is a possibility that it is 

 due to nothing more fundamental than the 

 peculiar aggregate state of the cytoplasm at 

 the time. 



Likewise in the ctenophore egg, which 

 also undergoes determinate cleavage, there 

 are evidences of regulatory development in 

 the formation of the aboral sense organ and 

 the gastral pouches (Driesph and Morgan 

 1895). Chun's (1892) observations on the 

 later stages of development indicate that 

 the partial larvae which develop out of 

 separated blastomeres are transformed by 

 a mode of "postgeneration" into whole 

 ctenophores. 



While the three best known types of 

 "mosaic" eggs thus show that "determi- 

 nate" cleavage may lead to an embryo 

 having some lability with respect to differ- 

 entiation, an examination of the so-called 

 "regulation" eggs shows, on the other 

 hand, a considerable degree of localization 

 within the cytoplasm. In the Hydromedu- 

 sae {Clytia, Laodice) single blastomeres of 

 the eight-cell stage may give rise to a whole 

 organism (Zoja 1895-6). However, in 

 many hydrozoan eggs there is a concentric 



5C/. Daleq (1938), p. 104. 



stratification with definite ectoplasmic and 

 endoplasmic layers, so arranged that each 

 of the early blastomeres receives both sub- 

 stances, and it is only later that cells of 

 pure ectoplasm or pure endoplasm arise by 

 cleavage parallel to the surface. 



It was with reference to the sea-urchin 

 egg that the complete totipotence of indi- 

 vidual blastomeres was first maintained. 

 This interpretation goes back to Driesch 

 (1891, 1892), who found that blastomeres 

 isolated in the two- or four-cell stage, while 

 segmenting as fractions, close over in the 

 blastula, gastrulate normally and give rise 

 to normal plutei of reduced size. Later 

 Driesch claimed that even single blasto- 

 meres of the 32-cell stage would gastrulate 

 and he showed conclusively that two eggs 

 or blastulae could be fused together so as 

 to give a single normal embryo. However, 

 the sweeping conclusion drawn by Driesch 

 (1894, 1929") from the above facts, that 

 "the prospective value of any blastula cell 

 is a function of its position in the whole" 

 does not hold. The experiments on which it 

 was based were not sufficiently controlled 

 to warrant the conclusions, nor were the 

 methods available at that time adequate to 

 carry out the requisite experiments. 



Driesch 's generalization implies that cells 

 in their differentiation must be subjected 

 to the action of surrounding cells, their 

 internal environment. That is, the term 

 ' ' position ' ' means their location with refer- 

 ence to other cells. However, the gastrular 

 invagination begins at a certain point on 

 the spherical blastula, and surrounding 

 cells are carried in with it ; this must mean 

 either that there is something peculiar to 

 the cells at that point, which is contrary to 

 Driesch 's hypothesis, or that the point is 

 determined by environmental factors of an 

 accidental nature. The latter alternative 

 is highly improbable. 



It remained for von Ubisch (1925, 1925a, 

 1931, 1933, 1933a, 1936, 1939) and Horsta- 

 dius (1928, 1935, 1937, 1938, 1939), by 

 means of ingenious devices, to explore the 

 ways in which blastomeres act upon one 

 another in gastrulation and in various proc- 



6 Op. cit., p. 55. 



