48 Edmund B. Wilson. 



It may be pointed out that not one of these larvae shows a 

 fully developed post - trochal region, though 91 and 92 arose 

 respectively from 2- and 4-cell stages that show nearly the normal 

 proportions and must have been produced from nearly normal 

 trefoils. This may seem to contradict the conclusion, drawn 

 above, that the predetermination of the lower polar area is not 

 quantitative ; but a similar reduction sometimes exists in this region 

 when the whole polar area is present (as in Fig. 85), and I do 

 not think a trustworthy conclusion can be drawn without addi- 

 tional data. 



I may add that after a large number of unsuccessful attempts 

 I obtained two nearly normal dwarf trochophores from frag- 

 ments of the unsegmented egg of Patella. One of- these, which is 

 about half the volume of a normal larva, clearly shows the cells 

 of the prototroch. In the full - sized normal trochophore of 

 Patella the prototroch, as may be seen with the greatest clearness 

 in total preparations, consists of a closed principal ring of cells 

 that vary in number (as seen in optical section) from 19 to 21. 

 In the dwarf the cells are more variable in size and less regularly 

 arranged, but on the average as large as in the normal individual; 

 equatorial optical section of this larva shows 13 cells in the prin- 

 cipal ring. 



VI. 



OBSERVATIONS ON FRAGMENTS OF THE FERTILIZED EGGS AND 

 ON THE ISOLATED LOBE. 



Extremely interesting and curious results are obtained by a 

 comparison of the behavior of fragments of fertilized eggs, and of 

 the isolated polar lobe, with that of fragments of the unfertilized 

 eggs described above. 



{a) The behavior of fragments of fertilized eggs obtained 

 before cleavage. — In order to malce sure that the eggs were fer- 

 tilized the operation was delayed until one or both polar bodies 

 had been formed, and the egg was then cut as nearly as possible 

 horizontally, so as to separate the lower polar area from the 

 nucleated part. As already described, if this operation be per- 

 formed on the unfertilized egg, and the two fragments be fer- 

 tilized, both may, and frequently do, develop. When, however, 



