1. Non-nucleated fragments of the egg fertilized by one spermatozoon may segment and such pieces have been found as late as the 16-cell-stage. 2. Some of the fragments were large enough, judging from the results of the development of nucleated fragments, to produce “partial larvae”. 3. The spermatozoa of Echinus enter with the same difficulty the non-nucleated fragments of Sphaerechinus that they do the nucleated fragments and whole eggs. 4. The crossed larvae of Sphaerechinus and Echinus (and also Strongylocentrotus lividus and Sphaerechinus) show a great amount of variability in the skeleton. 5. Amongst the crossed larvae a large percentage of forms showing the pure Echinus-type of skeleton will be found. The same resultSeeliger has obtained. 6. The results taken all together do not exclude the possibility ofBoveri's experiment, but they show thatBoveri's conclusion rests on insufficient and unsound evidence.