Experimental Studies on Germinal Localization. 237 



sitlon, except that one of the groups of four has turned over 

 (doubtless during the removal with the pipette), so that the 

 upper group presents to view the outer, the lower one the inner, 

 side; while the rosette-cell is somewhat too large. Both the cases 

 figured represent the open type, which appears to be the rule in 

 the quarter cleavage. 



3. The half and quarter larva in Patella. 



The detailed study of the larvae derived from the 3^ or K- 

 blastomeres presents many practical difficulties. While the early 

 cleavage of these embryos is easily determined, the later stages 

 are exceedingly difficult to follow, owing to the shiftings of the 

 cells, the more or less complete closure of the embryos, and the 

 great number of defective or monstrous forms, I must admit that 

 as far. as Patella is concerned, and in some respects in Dentalium 

 also, the following account is far from satisfactory, especially in 

 regard to the most interesting question of all, that of the meso- 

 blast; but since I may have no opportunity to complete it at pres- 

 ent, I desire to record some observations which may at least open 

 the way for a more adequate study in the future. 



The most essential point has been recorded in my preceding 

 paper, namely, that in Dentalium neither the >4- nor the 34-blas- 

 tomere is able to produce a perfect dwarf larva; and, further, 

 that the AB and the CD halves show definite and constant dif- 

 ferences, the former lacking both the post-trochal region and the 

 apical organ, while both these structures are present in the CD 

 larva. In like manner, among the quarter larvae only the D- 

 fourth produces these two structures, which are entirely lacking 

 in the A, B or C-fourths. 



In Patella the corresponding comparison is far more difficult, 

 owing partly to the equal size of the halves or quadrants, but 

 more especially to the even greater difficulty of rearing the larvae, 

 which very frequently go to pieces during the late cleavage stages, 

 and invariably become irregular and monstrous during the sec- 

 ond day, and finally disintegrated before the larval characters be- 

 come clearly marked. My observations clearly show one point, 



