S6o PATTERNS AND PROBLEMS OF DEVELOPMENT 



larger, as normally, and the resulting larvae develop a prototroch but die 

 before attaining the vcliger stage. In general, isolated blastomeres cleave 

 as if other blastomeres were present, and may develop into defective 

 forms with or without cilia but with entoderm overgrown by ectoderm 

 (Crampton, 1896). 



The studies by E. B. Wilson (1904) give additional information on de- 

 velopment of isolated parts of eggs and early stages of the mollusks 

 Dentaliiim and Patella. In the living, unfertilized Dentalium egg two white 

 polar areas are visible, the intervening region being superficially pigment- 

 ed: the basal white area enters the polar lobe when that forms. Apical 

 pieces of the unfertilized egg, when fertilized, do not form a polar lobe; 

 the first two cleavages are equal; and cihated swimming larvae may de- 

 velop but almost always lack the apical tuft of long cilia. The basal piece, 

 when fertilized, forms a polar lobe, cleaves like the entire egg, and may de- 

 velop into a normal trochophore. Pieces isolated by apicobasal section 

 through the basal white area may cleave like whole eggs and give rise to 

 "nearly normal" trochophores. The apical piece of the fertilized egg de- 

 velops like that of the unfertilized; but the basal, nonnucleated piece forms 

 the three polar lobes successively without dividing. After removal of the 

 polar lobe in the two-cell stage, the following cleavages are essentially 

 equal, and 2d and 4d are no larger than other cells; and in the resulting 

 larvae the posttrochal region is absent or small and rounded and never 

 develops further, and the apical tuft is also absent. If the first polar lobe 

 is allowed to complete its normal cycle with return to the cell body and the 

 second lobe is removed, the resulting larvae also lack the posttrochal re- 

 gion, but the apical tuft is present. Larvae from the yl5-blastomere of the 

 two-cell stage or from ^ , 5, or C of the four-cell stage are similar, except in 

 size, to those from which the first lobe has been removed; but larvae from 

 the CD- or the D-blastomere, of which the polar lobe is a part, possess 

 both posttrochal region, usually "too large," and apical tuft. Wilson con- 

 cludes that the material of the lobe must be specific and the determining 

 cause of development of apical organ and posttrochal region. Centrifuge 

 experiments indicate, however, that the polar lobe of another mollusk, 

 Ilyanassa, represents activity of the cortex which is not displaced by cen- 

 trifuging and that altered distribution in the lobe of other cytoplasmic con- 

 stituents has no effect on its formation or on further development. Effects 

 of hydrostatic pressure in inhibiting lobe formation and cleavage point to 

 the same conclusion.'' If the lobe is cortical, the relation between the first 



' Morgan, 1935, 1936; Pease, 1940. See also F. R. Lillie, 1906. 



