64 Edmund B. Jf^ilson. 



evidence that the primary determining factors of development 

 are to be sought in the nuclear organization. The well-known 

 hybridization experiments of Boveri ('92, p. 469) and Driesch 

 ('98) on sea-urchins have shown that the earlier cleavage-factors 

 conform to the maternal type and hence must be predetermined 

 in the egg-cytoplasm; and up to the blastula-stage, at least, the 

 embryos remain of the pure maternal type. But the same ex- 

 periments demonstrate no less clearly that the nucleus begins to 

 affect the cytoplasmic phenomena at least as early as the late 

 (prismatic) gastrula, and according to Boveri's latest work ('03) 

 as early as the mesenchyme-formation, though the latter point is 

 disputed by Driesch ('03). It therefore appears possible, not 

 to say probable, that every cytoplasmic differentiation, whether 

 manifested earlier or later, has been determined by a process in 

 which the nucleus is directly concerned, and that the regional 

 specifications of the egg-substance are all essentially of secondary 

 origin. 



Another question, which has been often discussed, is raised 

 by these observations, namely, as to the relation in the regenerative 

 process between the moulding of the mass as a whole (which 

 falls under the general conception of Roux's "Umordung der 

 Zellen" or Morgan "morphallaxis) and the specification of the 

 individual cells. Like the facts determined by Fischel ('98) in 

 the ctenophore egg (following the earlier work of Driesch and 

 Morgan) those observed in Dentalium bring out with great clear- 

 ness the independence, in this case, of the two groups of factors 

 by which these are determined. It is a very noteworthy fact that 

 all the partial larvae that lack the lower polar area, whatever 

 their size or mode of origin, tend to assume the same form, and 

 all are alike devoid of further regenerative capacity. The larvae 

 arising from entire eggs after removal of the polar lobe only, 

 the CD half from which the second polar lobe has been removed, 

 the AB half, the A, B or C quarter, or an upper fragment, of 

 any size, of the unsegmented egg — all these typically assume 

 the characteristic pyriform shape with the trochoblasts surround- 

 ing the larger posterior end. This form, which results after 

 closure of the embryos and gastrulation, is essentially a prolate 



