82 



THE CELL AND PROTOPLASM 



to its axis. As the time interval between 

 fertilization and removal of the fragment 

 increases, the defects become more marked 

 and more specific. These statements are 

 based upon experiments with eggs of 

 nemerteans (Wilson 1903; Zeleny 1904), 

 Dentalium (Wilson 1904) and ascidians 

 (Dalcq 1932). One of the most striking 

 cases of defect after removal of portions 

 of the cytoplasm of an unsegmented egg is 

 found in the ctenophore, Bero'e ovata 

 (Driesch and Morgan 1895; Fischel 1903; 

 Yatsu 1912). In this form there is a 

 definite relation between the position and 

 the amount of the cytoplasm removed and 

 the number of rows of swimming plates 

 present in the embrj^o which develops from 

 the remainder. 



Experiments of this kind show that there 

 are regional differences in a given egg and 

 also differences between species of eggs with 

 respect to their germinal localization. 

 There is likewise variation in the extent to 

 which defects in the early stages of develop- 

 ment may be made up by the material that 

 is left, and this holds both for the unseg- 

 mented egg and the egg which has under- 

 gone cleavage. Driesch thought all cells of 

 the sea urchin blastula to be alike, as 

 measured by what they can do in the 

 course of development (prospective po- 

 tency), and that what they actually do is 

 a function of their position in the whole. 

 On the other hand, gastropod, annelid, 

 ctenophore and ascidian eggs have been 

 characterized as mosaics, in which the role 

 of the constituent cells is fixed from the 

 beginning and cannot be changed. Cleav- 

 age has correspondingly been described as 

 indeterminate or determinate and the 

 course of development in the respective 

 classes as regulatory or mosaic. 



It is obvious that eggs do differ in this 

 respect, but no egg is wholly in either class. 

 If mosaic development were the rule my 

 theme would end here, for subsequent 

 changes in the embryo would then take 

 place according to the initial structure of 

 the germ, and could not be modified by the 

 internal environment. 



In the egg of the molluscs, Ilyanassa 



(Crampton 1896), Dentalium and Patella 

 (Wilson 1904, 1904a), each cell when sepa- 

 rated from the rest gives rise to just what 

 it would, were it left in place in the embryo. 

 Here, then, the internal environment of 

 blastomeres may be assumed to have no ef- 

 fect on differentiation. Wilson found, how- 

 ever, that each fragment of an unfertilized 

 egg, when divided vertically and fertilized, 

 develops into a whole larva, thus showing- 

 regulation within the single cell. On the 

 other hand, removal of the polar lobe re- 

 sults in a deficiency of the apical organ and 

 the post-trochal region that is not made up. 

 The case of Tubifex, an oligochaete, which 

 like the gastropods and scaphopods has a 

 spiral cleavage and a mosaic development, 

 is nevertheless interesting in showing a con- 

 siderable amount of regulation under cer- 

 tain conditions (Penners 1924, 1925). 

 When the polplasm is divided equally at the 

 first cleavage, which takes place frequently 

 as a result of low temperature, both cells 

 contain teloblast material and the result is 

 a double embryo (duplicitas cruciata). 

 When the AB cell is killed by ultraviolet 

 radiation, the CD half, and probably also 

 the D-quarter when all three remaining 

 cells are destroyed, give rise to whole 

 embryos. The above facts imply consider- 

 able powers of regulation in these eggs with 

 highly determinate cleavage and also that 

 in such eggs the cells constituting a mutual 

 internal environment do normally restrict 

 the activity of neighboring elements. 



The ascidian egg, which according to 

 Conklin (1905, 1905a, 1911, 1931 ) holds to 

 a very rigid cleavage mosaic, shows con- 

 siderable lability before fertilization. It 

 may then be deprived of large amounts of 

 cytoplasm and still develop normally 

 (Reverberi 1931; Dalcq 1932). Even two 

 complete embryos, one of which is mero- 

 gonic, may sometimes be obtained from a 

 single egg. There is, however, indication 

 of bilateral symmetry at this early stage. 

 Evidences of regulation in the fertilized 

 egg have been found by v. Ubisch (1938), 

 who describes the development of a single 

 giant larva from two fused eggs. In em- 

 bryos derived from one of the first two 



