490 



SCIENCE. 



[N.S. Vol. XVII. No. 430. 



thus arise that are typically open on one 

 side, or in extreme cases form curved or 

 even nearly flat plates ; but all these forms 

 may ultimately close, gastrulate and give 

 rise to pilidia, though those arising from 

 the plate-forms appear to be always asym- 

 metrical or otherwise abnormal. 



These facts prove that in this egg, which 

 shows a typical spiral mosaic-like cleavage, 

 the form of cleavage is not essential to 

 normal development, since the egg frag- 

 ment segments as a whole, the isolated 

 blastomere as a fraction, yet both may 

 produce the same result. They prove, 

 further, that the factors determining the 

 cleavage mosaic are not definitely localized 

 in predetermined germ areas before forma- 

 tion of the polar bodies, but become so 

 localized in the period between the begin- 

 ning of maturation and the completion of 

 the first cleavage. Sections show that dur- 

 ing this period a polarized segregation of 

 material takes place. Comparison, espe- 

 cially with the segregation of material 

 occurring at the corresponding period in 

 the eggs of sea-urchins and moUusks, as 

 described by Boveri, Lillie and Conklin, 

 and with the results of Boveri 's experi- 

 ments, leads to the conclusion that this 

 segregation of material is the immediate 

 cause by which the cleavage factors are 

 localized and the form of cleavage de- 

 termined. Every differential cleavage is 

 probably preceded by analogous segrega- 

 tion of cytoplasmic materials, which not 

 only form an important factor in deter- 

 mining the form of cleavage, but probably 

 are a factor in cell-specification. Cleavage 

 thus plays an important part in differen- 

 tiation and localization, not as a direct 

 cause, but indirectly as a means of isola- 

 tion of different materials. The cleavage- 

 mosaic thus becomes a mosaic of such ma- 

 terials and of corresponding developmental 

 tendencies in the individual blastomeres. 

 This mosaic-like character is, however, not 



due to the preexistence of corresponding 

 areas in the unsegmented egg, but to a 

 progressive process that is essentially epi- 

 genetic in character. The primary egg- 

 polarity certainly, and perhaps some other 

 characters, such as bilaterality, preexist in 

 the immature egg, but other cleavage fac- 

 tors are localized by a progressive process 

 in which cytoplasmic movements are a 

 leading factor. 



Merogony and Regeneration in Benilla: 

 Edmund B. Wilson, Columbia Univer- 

 sity. 



1. When fertilized eggs of Benilla are 

 cut into two or more fragments during the 

 earlier period preceding cleavage, one of 

 the fragments may develop into a dwarf 

 embryo, segmenting at once into eight or 

 ten blastomeres, like a whole egg of dimin- 

 ished size. During the later period, after 

 division of the cleavage nucleus, two or 

 more fragments may develop; but in this 

 case each fragment divides into a smaller 

 number of blastomeres than those produced 

 by an entire egg, the total number being 

 approximately the same as those produced 

 by a whole egg. Cleavage in this egg 

 therefore depends not upon the presence 

 of a certain number of nuclei, but upon 

 the attainment of a critical stage by some 

 other progressive change. The egg frag- 

 ment may give rise to a planula, and ulti- 

 mately to a young colony, entirely normal 

 in its structure and proportions, but of 

 diminished size. In this way may be pro- 

 duced dwarf colonies down to about one 

 fourth the bulk of the normal; but, like 

 the full-sized colonies, they do not produce 

 more than a single pair of buds under the 

 conditions in the aquarium. Budding in 

 Benilla is, therefore, not dependent upon 

 the amount of material present, but is a 

 process entirely analogous to the formation 

 of organs in the ontogeny of a single indi- 

 vidual. 



