352 PROCEEDINGS OF THE ACADEMY OF [July, 



has found that when the polar lobe of Dentaliuvi is removed the resulting 

 larva lacks the post-trochal region and probably also the mesodermal 

 bands. Both of these authors found that following the removal of the 

 lo])e the four quadrants of the egg were equal in size. It cannot be 

 assumed that the size of the lobe is proportional to the size of the 

 somatoblasts 2d and 4rf, or to the size of the post-trochal region, or to 

 the quantity of mesoderm which is formed later. In Fulgur, Crepidula 

 and ail other cases in which the quadrants are nearly equal in size, the 

 yolk lobe is much smaller than the post-trochal region or the mesento- 

 mere 4c?, or even the mesodermal bands; while in Illi/onassd, Tritia, 

 and other forms in which one of the macromeres is much, larger than 

 the others, the yolk lobe is much larger than the cell 4rf. On the other 

 hand the lobe is usually associated with unequal division of the 

 yolk, and its size is proportional to that inequality. Therefore I 

 believe that the real significance of the yolk lobe is to be found in its 

 relation to equal and unequal cleavage. 



Since the blastopore forms at the vegetal pole in all gasteropods, so 

 far as known, unequal division of the yolk must lead to unequal growth 

 on the part of the blastoderm: if the posterior m^cromere is large the 

 overgrowth must be greater in a posterior direction than in an}^ other; 

 if the anterior macromere is larger than the others, as in certain 

 opisthobranchs, the blastoderm must grow more anteriorly than 

 posteriorly. When all the macromeres are equal the overgrowth must 

 take place equally in all directions, though the case of Fulgur shows 

 that it may be accelerated at one time in one portion of the blastoderm 

 and at another time in another portion. 



The most striking difference Vjetween Fidgur and other gasteropods is 

 found in the manner of this overgrowth, and consequently in the manner 

 in which the different organ bases are carried from theu" point of origin 

 to tlieir definitive positions in the embryo. In both the point of origin 

 and the definitive position of the various organs there is fundamental 

 agreement between Fulgur and Crepidula, but in the manner of passing 

 from their point of origin to their final position there is marked dissimi- 

 larity. In Crepidula the region of greatest growth of the blastoderm 

 lies on the dorsal side between the anlagen of the cerebral ganglia and 

 the shell gland ; in Fidgur it lies anterior to the cerebral anlagen. In 

 Crepidula, as a result of this condition, the cerebral ganglia are carried 

 forward around the anterior end of the embryo vmtil they come to lie 

 anterior to the mouth on the ventral side. In Fulgur the same ganglia 

 are carried backward around the yolk until they finally reach the same 

 position on the ventral side of the embryo. 



