552 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



toptcrus. This result, like Morgan's centrifuge experiments, indicates that 

 the lobe is a cortical effect, associated with cytoplasmic division but 

 largely independent of the mitotic apparatus (Pease, 1940). Experi- 

 mentally the first lobe can be made to fuse with either of the cells of the 

 two-cell stage in the mollusks Dcntalium (Schleip, 1939, p- 208) and 

 Ilyanassa (Morgan, 1936) and in the annelid Sahellaria (Novikoff, 1940, 

 see footnote 8, p. 559), and the cell receiving it develops like a CD cell. 



B 



C D 



Fig. 17s, A-D. — Early cleavages of Denlalium, to show the three successive polar lobes, 

 Li-Lj, and approximate boundaries of pigmented zone, indicated by dotted lines (after E. B. 

 Wilson, 1904). 



The earlier, purely descriptive studies of cell lineage in forms with spiral 

 cleavage led to a further development of the hypothesis of "organ-forming 

 germ regions" advanced by His (1874). The definite cytoplasmic localiza- 

 tion, "precocious segregation," of different organ-forming substances and 

 their accurate distribution by cleavages was postulated, and the concept 

 of homology was applied to individual cells in the cleavage pattern, cer- 

 tain small cells in some forms being regarded as vestigial or rudimentary. 

 According to these views, development in forms with spiral cleavage is a 



